I would love for Pichu to make a return. Sure, he sucked in Melee but he was so fun to play as that I really didn't care. I don't think he's very likely, mostly because six Pokemon reps is already a ton, but the argument of "this character has to be relevant" is thrown out the window now because Roy got in and he's far from relevant. He also represents Generation II, so 4 out of the 6 generations would be represented by playable characters.
Personally, if he does miraculously come back, I think all recoil attacks should do just 1% to him. Also, Agility and Skull Bash should not have any recoil because they're not electric type moves. Sure, he will need some more serious buffs to put him in line with a majority of the cast, even though he was a joke character, but I think that is a good start. I could see Pichu being somewhat decent in Smash 4, assuming he gets a ton of buffs. Since there's the rage mechanic, he could just boost his damage to to rage for KOs if he wants.