As many others have said, Young Link and Doc seem like a bit of a waste at this point, since so many of their defining attributes were rolled into TL and Mario. That said, I know there are people out there who love both characters. TL's one of my favorite now, but I still love Young Link, and there is a part of me that'd be really happy to see him come back. It'd just feel kinda silly to have 3 of the same characters. Considering how much time it would take the PM BR to make him, it doesn't seem like a good use of their resources and energy.
However, Pichu's kind of interesting. As far as I know, absolutely nothing about Pikachu was changed to resemble Pichu. There's definitely some interesting potential there. I'd kind of love to see what'd happen to Pichu if they took away his self-destructive playstyle. Keep him as a really fast, light, weak version of Pikachu, just don't have him hurting himself during some of his attacks. I wonder if that, combined with some tweaks to make him a better character overall, would be enough to actually make him a legitimate choice?
But maybe that's not the point - maybe the point of Pichu is to be a ridiculous character that's kind of humiliating to lose to. In that case, I guess it's a question of whether the PM BR would embrace spending all that time and effort on a joke.
I'll be honest, though, I'd be really happy if he came back. I played him all the time in Melee, just to troll my friends/cousins.
Honestly, I'd be happy if Young Link and even Doc came back, too. I know it's silly and sorta pointless, but I do miss those guys. =) Ever since I fell in love with PM, I've been playing Melee less and less. Heck, the last time my bro and I booted it up was just because he missed Roy, and wanted to play as him again! With Roy coming back and PM continuing to be a better and better game, it's a little sad - it's like I'm finally saying goodbye to Melee.
Wah, I got pretty off topic here. I guess tl;dr - I think it'd be cool if the old Melee crew was reunited, even though it might be redundant to have them.