I'm not too sure. There's two main options for me.
Pikachu is extremely good in my opinion. With pancaking on the ground after most landings, having most of his attacks safe on shield, his vicious combo game, his nearly not existent disadvantage state, the only thing holding the character back is a lack of results, but ESAM is pushing this character to his sky-high limits. Now, I'm not a fan of gutting things out of characters because they are good, and as much as I dislike Pikachu's aerials, I wouldn't go so far as to say that they are overtuned, and even than, every character in this game has silly attributes about them.
The first thing that comes to mind is to make Pikachu lighter. As it stands currently, I think there is no reason why Fox should be lighter than Pikachu, with Fox being bigger, easier to hit, and more prone to off-stage deaths than Pikachu. The second and last nerf I would make is to make Pikachu bigger and to have him no longer pancake on landing. What pancaking means in this term is that the character in question squishs onto the ground almost like a pancake upon landing onto the stage and becomes nearly impossible to hit. Pikachu is already very small and is at times smaller than his pre-evolution, Pichu, because of Pichu's ears, and I do not feel like Pikachu needs to have pancaking on top of that. That is all I would do, Pikachu still gets to have his silly stuff, but now his weaknesses are a bit more glaring, allowing him to have to be more careful.
The second option would be the Phantom Thief himself, Joker. Undoubtedly a top tier character, his comeback mechanic of Arsene, his great tools in neutral game, the many different varities of Gun, and Joker had the best results out of any character during the second Season of the PGRU. Now, once again, I do not want to extremely nerf this character, I just think that he could be toned down just a tiny bit.
One thing I think could be is to nerf Gun, for the most part just Down Gun. I think that making the bullets a projectile rather than a very long disjointed hitbox is not an awful idea, and would make it more fair to other moves in comparison to say, Mega Man's Down Air, which is a projectile and thus could be reflected, but has a spike hitbox. so it's still very good. There also has been discussion about nerfing Joker's counter, more noticeably the Arsene variant, Tetrakarn. I would say making the hitbox on the move when you counter it would not be a bad change, especially considering the size of the move is extremely huge. I also think that Joker shouldn't get 20% of his Arsene meter upon losing a stock. He does not need it.
Those are my personal takes on the matter, personally, I believe we should for sure buff more than nerf. Thoughts?