Just spent the afternoon looking through the cards and thinking of what cards could use some tweaking in a future rebalance. Most of it is buffs to underused or unused cards, both vanilla and Plus. But there's some nerfs to the more obvious suspects. All sorted by card type for easier parsing.
SCs
Spoiler
Teifu: He's mostly fine but -3 HP feels bad so it would be nice if he had an extra HP or two.
Orland: I also think he's mostly fine but swapping his sword bonus to ally bonus would make for a pretty good QoL so he wouldn't be stuck as a one trick pony. Though I imagine ally bonus on him might need to be 1/3rd instead of half? Just because its something you can somewhat control, unlike Teifu's enemy bonus.
Reiz: I don't really have any suggestions myself, but the idea that's been thrown around of giving her 5 AP and Weak Spot 4 seems like a nice way to make her less matchup dependent.
Items
Spoiler
Flowen's Sword: I feel like this card was made super underwhelming because the shield has Full Heal, but the Link: Return of the shield doesn't feel worth playing the sword to get and the sword's low stats also doesn't make the Link: Snatch an interesting proposition either. Though I also think giving this more AP wouldn't help and would just step on the basic Sword's thing, so my idea here is making the HP 4 so its in between Sword and Huge Battle Fan in durability. Giving it +1 TP and a yellow link could also be a neat idea to make it more unique.
Bloody Art: The abilities on this are kind of interesting but functionally don't do much because of it having such low HP, and in most cases Cross Scar is just a MUCH better option even with that one having Steady Damage. Sword bonus being 1/4th largely means it'll often get nothing outside of multiple players having dedicated sword decks. Lastly, Artifice means this card is even more fragile than its measly 3 HP suggests. In the end I'm not really sure how to tackle this one without reworking the card completely, upping its HP so it can survive a round or two would be nice but it would end up stepping on the toes of Akiko's Wok Set as the "High HP dagger."
Photon Claw: I didn't realize this at first but this card has no links, and the combination of that with 1 HP and no other stats feels kind of harsh for what this card does. Heavy Pierce is nice and all but Hand Disrupter doesn't tend to do much in most scenarios. But seeing as pierce can be such a powerful ability I think this having no links is fair since its a 2 cost, but having like 3 or 4 HP would make it at least have a chance of seeing more than one turn in play.
Silence Claw: Honestly, this card is one detail from being fine, just change its Steady Damage from 2 to 1. Major Death is a powerful ability but Big Swing and the claw's low stats already help tone it down to where regular Steady Damage is enough. An alternative is to make the claw have high stats as well because Steady Damage always makes one ask the question of "Does this kill the opponent faster than it kills me?" and in this case Silence Claw 100% kills you faster than it does your opponent. That being said, I personally making it into stat stick would run counter to the niche Major Death tries to push it in.
Gobooma's Claw: Orange top color why?
Toy Hammer: I get that this is supposed to be a joke weapon but, uh... 1 HP, no links and only Random AP -2 means the ceiling of this card is being a 1 HP +4 AP item, still with no links. Using Slash with a Saber gets you there with better range and other stats. Changing nothing else, give it more HP and a full random AP so it would at least make it fun in a gambling deck.
Photon Launcher: One of several cards where they overcompensated hard for one upside, in this case heavy rampage. I'd say just give it Gun AP Count 1/2 instead of 1/3 and call it a day. It gets to have a bit more AP but its still a 3 HP 3 cost item with Big Swing. (Honestly, X AP Count abilities seem undertuned in the vast majority of cases.)
Guilty Light: 0 AP 3 cost with Freeze 6 and Snatch 1/3. I think giving it +1 AP would be fine but otherwise not much to say.
Red Scorpio: Holy Ray but with 2 less HP and no para because it has multi-targeting, as it can barely get any AP with the Gun AP Count being 1/4. Not really sure what to do with this one but 3 HP on a 4 cost feels awful. (I still don't have any of these, by the way.)
Branch of Paku Paku: Why does this have no yellow links and no TP Power? Its normal for a 1 cost to be fairly weak but this is a contender for worst card in the game. (Not sure if TP Power would work with the Cane TP Count but if it did that'd at least be a funny gimmick.)
Plantain Leaf: Having no yellow links too is one thing, but why only 1 HP? Give at least 3 so it maybe survive a hit after using its AP Silence.
Dancing Hitogata: Nothing overly bad with this one but it kind of feels like a worse version of Magical Piece. both have Random TP and 10+ HP but this costs 6 and has no AP to use with its shotgun range. Maybe give it 1 or 2 AP just because its fairly underpowered for a 6 cost?
Divinity Barrier and Shifta Merge: Probably a weird idea, but maybe exchange their HP values? Its weird that the purely defensive one has less HP than the offensive barrier. And it'd make Kasami Bracer just a smidge more worth it for HUs.
Garuda: Maybe a bit more HP seeing as the mag inherently does nothing but make you fly?
Chu Chu: Definitely could have more HP. As its a 3 cost that does nothing for 8 whole turns and then explodes, which does cause you the normal 1 damage of an item being destroyed. Alternatively, give it Filial so doesn't actually hurt you when its destroyed.
Creatures
Spoiler
Barble: Where do I even begin with this one? A guard creature with one of the highest Unfilial value in the game and what do you get for it? Somewhat decent HP but zero offensive presence. Even in a dedicated NA deck I'd much rather play Dubchich if I absolutely want a 3 cost guard or Gi Gue if still want an NA guard. There's a few ways to go about making it not as awful; could change nothing but drop it to 2 cost so the higher Unfilial is justified. Could drop the Unfilial to 1 so its the worst overall guard but hurts you less without being walled behind a high cost. Lastly, its offense could be made to not be garbage tier, but that's probably the least interesting option seeing as you often don't want your guards to be on the frontline anyhow.
Barbarous Wolf: Not really much to say, just give it +1 to either AP and/or MV so its not as bad as a lone wolf.
Gulgus-Gue: Its Leader ability essentially does nothing due Gulgus' own ability so perhaps removing both it and Weak Spot would make the creature a sidegrade to fellow 4 cost NA Hildebear. Gulgus-Gue has less range, 1 extra HP and -2 AP/TP(and no yellow link) so it would a weaker but more mobile threat.
El Rappy: A fairly unremarkable 3 cost that can't use ACs. Its ceiling is hitting for 7 which is certainly better than Toy Hammer so I'll largely say the same thing about it; just give a bit more HP so it can actually have a chance to live a hit. Something like 5 HP would be fair for a 3 cost.
Pal Rappy: Basically Al Rappy but it costs 1 more, has 1 less TP and can potentially sacrifice itself to boost ally TP. Maybe give it like +1 HP and/or AP just because its a 3 cost.
Mothvert: In most cases its a Mothmant but worse, giving it a singular point of base AP would make it a sidegrade rather than a downgrade.
Gee: Largely fine, but waiting 8 turns for 5 exp feels a bit long, reducing it by one round(AKA 6 turns) would be fine, I think? But not a big deal either way.
Gi Gue: Give this thing an Unfilial value, please. While a 6 cost should be fairly strong, Gi Gue is just unfun to fight against and it reflecting full damage when defending feels unfair.
Guil Shark: The card is cool but 5 AP is already a lot for a 3 cost and Group just tends to make it punch above its weight too much. As such I think it should have a higher Weak Spot value like 4, just so it won't obliterate SCs despite not being meant to.
Melqueek: I don't think this card is super weak but it suffers from trying to be an inbetween of Pouilly Slime and Merlan. I guess giving a +1 to AP would at least help it with the rounding as 3 AP with Slayer means its just doing 4 to majors without outside buffs or ACs. But can't really give too much because haste is a dangerous ability.
Pofuilly Slime: Poor thing really struggles to have any offensive presence due to Copy Half rounding down, meaning most of the time Evil Shark does the same job better and the only time where the slime would do better its fighting against something where the 2 HP it has over shark are irrelevant. So the one time slime can be considered better is if swarming quickly with Breed. Frankly though, I have no idea how to make slime a more interesting option without either reworking it entirely.
Merillia: Removing HP Change from this would be a minor but welcome buff.
Meriltas: Could use 1 or maybe 2 base AP just so it can do anything while alone as it lacks a yellow link to use its TP. Giving it a yellow link could be an alternative but probably a bit more risky.
Dubchic: As I've been saying for most things with a X AP Count 1/2, giving it 1 base AP would at least allow it to do something when its alone.
Gilchic: Machines already have two other "do nothing until it blows up 8 turns later" that literally can't attack, so maybe it could be interesting to give this one the ability to be a low cost slapper and it has Stall so you may reasonably want to play it near enemies. I'm thinking that upping its HP to like 4 and its AP to 2 would be fair.
Sinow Gold: Simple QoL, remove Unfilial 1 from the Plus creature that has no obvious reason to have it.
Merlan: Reverting this card's AP to 4 would be nice as its kind of overtuned at 6. Maybe 5 for a compromise.
Dark Gunner: Reducing the Steady Damage from 2 to 1 would be a nice QoL, SD2 is really harsh even with an Arkz.
Delsaber: Really suffers from being at the same cost and type as Merlan, on top of X AP Count abilities being largely undertuned as a whole. Could give it a small boost to base AP or swap the AP count to give it a static value that's at least 3 so Drain functions. Maybe give it more TP for the laughs since it actually has a yellow link.
Dark Bringer: Haven't see this one in a while since we kind of soft-banned it, but either upping its cost to 7 or lowering its stats would be fair changes to make it less obnoxiously overtuned. Off the top of my head a statline for it to remain at 6 could be 14 HP, 5 AP, 2 TP and 2 MV.
Ill Gill: Several things could be done to make this card less absurd, if upping Dark Bringer to 7 cost this card could take its place as a 6 cost. Otherwise, I think reducing its stats and increasing its Weak Spot value would be welcome. A statline I could see is 9 HP, 8 AP, 1 TP and 3 MV with Weak Spot 5. (Why the heck does this thing even have TP to begin with?)
Actions
Spoiler
Chaotic Attack and Cross Slay: Both of these getting an additional AP would probably help these see more play. Chaotic Attack's targeting is quite narrow in use cases compared to Slash or Stab, and Cross Slay can't give +2 AP if Chaotic Attack also gives +2. Alternatively, could make Cross Slay target in an X shape rather than a plus shape and leave Chaotic Attack at +2.
Berserk+: This doing fixed damage equal to half the user's current HP is kind of underwhelming given it kills the user afterward. Its usually better to use regular Berserk or pay one more point for Kamikaze Attack to get more damage since that one adds half HP to your normal damage AND its a Round Slay range instead of single. No idea if it would be possible for this to do damage equal to attacker's HP, or half of max HP.
Combo: This is essentially a worse version of Unit Blow and there's not really a way to make it viable when they both share the same cost. But Unit Blow is perfectly fine as a 2 cost and reducing Combo's cost to 1 makes it a purely better version of Attack. And I don't really see this happening unless Attack also gets buffed but that's an extremely difficult card to buff without making it unbalanced.
Knock Out: Hold is a pretty broken status effect because it disables using defense actions and I feel paying 3 for a guaranteed infliction of the status barring specific defenses being used is too good even at 3 cost. There's several way to make KO less obnoxiously strong, introducing a roll of 3+ like other status ACs would bring it back in line. Alternatives are making the AP =0 instead of +0 so you have to sacrifice at least some damage for it. Lastly, the cost of the action could also be increased to 4 but I think that would be the worst way to go about it.
First Attack: I think this could reasonably go down to 2 cost as at 3 its kind of a hard sell unless you're specifically bypassing Action Disrupter.
HP Defense: Admittedly this isn't a card I have tested at all, but it seems so narrow that it could possibly drop to 1 cost.
Exist and Defense 7: I think Exist is entirely fine at 4 but does end up being a better version than Defense 7 in a lot of cases but seeing how difficult it is to slot in and play Exist maybe, just maybe, Defense 7 could go to 3?
Assists
Spoiler
Dice +1: Wishful thinking, but maybe this could get an inbetween of original and later balance and go to 2 cost?
Battle Royale: Given this assist's very short duration of 2 turns, perhaps either its cost could be reduce by 1 or its duration increased? As it is, you need to be in 2v2 for your side to really benefit from it at all. As it costs 4 and thus you'd need for it to give you at least 4 attacks for it to be worth it.
Saber Dance and Bullet Storm: Both of these cost 3 while the creature ones all cost 2 and outside of 2v2 HUs are usually better served by Brave Wind unless they absolutely need to overwrite an opponent's assist. Cane Palace is unique in that it gives TP instead so it doesn't really have an overlap.
Influence: This assist is in a weird spot, it costs 5 and the most common use case is swarm decks who'll be better served by March of the Meek, but I think this works with technique damage too? 5 seems to be a lot for what this does either way, so possibly reducing it to 4 instead?
Heavy Fog: I feel like this lasts too long for how debilitating it can be, maybe reducing it from 8 turns to 6 could be okay? This isn't a huge deal though.
E Cards
Spoiler
Toy Rappy: This kind of costs too much for what it does, especially for an E card. It could use either some AP or a cost reduction.
I might come back and edit some of these ocasionally but for now that's my thoughts on the matter, feel free to pitch in with your opinions, its always interesting to see. In the end this is mostly me just rambling rather than some hard suggestions.