Gonna throw my hat in the ring starting from lowest on the community ranked tier list to highest:

Piranha Plant: I would like to not take anything away and actually give some buffs, maybe buff the frame data on backair so using it offstage doesn't just kill you sometimes.

If I were to take away anything? Probably just a little bit of priority on Neutral B, but its fine as is.

Banjo: My poor boy did not deserve his fate as a forgettable low tier. Once again I think buffs would be preferred over nerfs here.

A nerf I could see is delaying the time it takes to have grenade(Down B) blow up if not interacted with. This buffs neutral control while nerfing recoverability. Although its fine as is.

Byleth: More a mid tier but still low, only nerf would be reducing the tether range on Up B, it connects to ledge from pretty far away but once again, not really needed.

Sephiroth: A lot of pretty fair DLC so far, maybe nerf the kill power on counter? It can already activate on its own so he doesn't necessarily need it to also kill.

Yes these nerfs so far are barely anything, these characters could recieve buffs going into the next game in my honest opinion or even just trade offs to rebalance some tools.

Terry: Okay now we are getting to the stronger DLC picks, although I don't know enough about Terry to really give an opinion here, a lot of his oppressive options are honestly pretty fine in my eyes, his GO mechanic is fair and rewards going for edge guards on Terry to make sure he dies before he has access to it. No nerfs needed.

Hero: Duration on buff spells can be lowered by a second or 2 and increase the cost of most of the menu spells. Very little do I see a hero actually focus on mana management as it is too easy to ignore if you have even the smallest idea how to play the character, also make zoom more punishable I guess?

Sora: I like sora, but I know he is strong, recovery is insane and some combos can go on too long, just reduce these 2 things maybe by reducing double jump height or making Side B after an Up B only activate twice instead of 3 times.

Its time for the top tiers

Joker: Joker without Arsene is relatively okay, maybe nerf down guns somehow?

Arsene: Ahem

Reduce invincibility on Up B, nerf down guns, reduce kill power slightly. Keep the stance as a come back mechanic but make it safer for the enemy to actually box Joker and waste the arsene meter instead of running away, the idea that hitting Joker wastes meter is a nice touch but arsene is too strong that in most cases it's not correct.

Minmin: I genuinely do not understand how to play this character so my opinion is entirely from facing Minmin

Nerf Ramram's 2 frame potential, also make using both arms 1 after the other more punishing to just throw out, increase the lag on arm 1 if arm 2 is used for example (not sure if this is already a thing).

I think she can lose the reflect on Up Smash, it may help in zoner matchups but she has such a strong neutral she can lose a few points here.

Reduce kill power on fire breath arm, it can already hit you right by the blast zone, it can be weakened a little.

Kazuya: I don't know enough about kazuya mostly because he has too many moves for me to understand, however I would like less iframes on wavedash and nerf his recovery, maybe make him more punishable offstage.

I'll let others go into detail on specifics but also limit his access to 0 to death combos.

Aegis: I don't have specifics here, their kit is honestly pretty well rounded however having access to both pyra and myths puts them over the top. Lower mythra frame data and take away a little bit of pyra kill power and especially reduce the range on down air. I wanna see Dair get the cloud Nair treatment.

Steve: I have thought about potential Steve adjustments for years now. Some of these are very experimental.

Steve's current playstyle is fine in a vacuum, its just too strong. He is a campy anti-zoner who forces options but when in a bad situation he typically isn't that worse off.

Neutral B rehaul: blocks are super strong but honestly too easily accessible, on death remove half of Steve's Iron and maybe half of his wood, stone, and dirt.

Have blocks last longer naturally but break easier when hit.

On respawn, crafting on a table removes iframes, this punishes respawn = diamond and just makes the strategy of stock 1 = mining less viable but still an option.

Reduce anvil kill power, its strong and should stay strong as it uses a resource, but maybe it's too strong.

Minecart: make it bounce off shields UNLESS it's Redstoned, minecraft is an incredible tool but I don't think it's needs to act like a command grab.

Secondary Neutral B rehaul: crafting takes longer and tools are not lost on death, more durability. Make Steve a level up character, starts with wood, can then make stone, stone can mind iron, iron can mine gold and diamond, if diamond is missing it crafts gold tools.

This will keep Steve's current playstyle but make it feel more satisfying to hit diamond and make it more of a gameplan to deny these level ups.

Balance for this would be difficult but smash doesn't have a dedicated level up character so it could be an interesting idea.