I'm not a big fan of the third party characters, cuz Melee's case says on the front:
Nintendo's Best in 4-Player Action!
Sonic and Snake are from other companies, so it kinda violates the original premise.
Other than them...
Young Link/Wind Waker Link: No thanks. Young Link is too similar to adult Link, and WW Link would look outta place (save that for someone like G&W).
Dr. Mario: No. Luigi is enough when it comes to Mario clones.
Pichu: Need I explain?
Roy: Unpopular and too much like Marth.
Tingle: He's popular in Japan so he stands a chance. But WHY?
Hideo Kojima supposedly litteraly begged to have Snake included... bazillions of fans have wanted Sonic since Melee came out, Yuji Naka wanted Sonic included, and Sakurai did too.
But Sonic and Snake reguarly appear on rival consoles, which is what I don't like about having them appear in an all-Nintendo style game. I mean, if Rare was still with Nintendo, I wouldn't mind seeing Banjo, even though he'd have been second party, because he'd still be considered a Nintendo-only character.