I bring up the size of the roster solely because if you're looking for a challenge, there are a lot of characters to look through so they can be found. Peach, Ryu, Pikachu, are all really challenging characters to play at a high level, maybe go with them.
Personally, I think characters like Pichu, Young Link, and other badly made clones were excusable back in Melee times because there was very small development time, and a very small roster, so roster padding made sense, but now there's no need, and we should really be focusing on getting new, unique characters as time goes on. That's not to say efforts can't be made to make a character like Pichu unique, but seeing that Sakurai actually tries to represent Pokemon with moves from their games, I don't exactly see the point.
I want characters that bring something to the table. IC's do that, Inklings can, if they change his specials Wolf can too. I can think of so many unique playstyles that will never be represented, because we're so focused on bringing back/in boring characters that have so little to offer.
What would you buff? The reason a lot of people like Pichu, is because Pichu is so disrespectful because he's so bad. Taking that away takes away the central point to the character. Not only that, but being somewhat self-destructive is kind of it's thing; Sakurai seems pretty loyal to Pokemon gimmicks, and I don't see that changing.
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If I'm to suggest any Pokemon; I'd say Zoroark, Sceptile, and Genesect would all be great choices. I don't see why we have to be hung up on Pichu for.