I think that none of them should exist for these reasons.
1. I think that Roy should never have existed, being less main in Fire Emblem then Ike. Also for the most important reason, they made him just as Agile as Marth. Having mainly different moves he should be on a different weight area.
2. Dr.Mario the biggest reason is obvious: He is a 2nd copy of Mario. You cannot have Mario, Luigi, Dr.Mario. Plus he was all the same except for the fireball move, which the idea of firing pills sucks. He should either have an entirely different move or a
well thought out similar replacement one.
3. Young Link is a mistake, he is basically the same as Link. Which you would think the idea of a copy would need something more different the fire arrows. Toon Link is very different with agility and can easily top normal link, and you wouldn't think that link was stronger as s kid, would you?
4. Pichu is a copy of Pikachu and a bad idea for SSB games. He needs to be more different then just being able to wall jump and have different smash attacks. And don't give me the crap that says that he is more agile then Pikachu. Pikachu is already agile enough. And he has nothing to value over Pikachu, considering Pikachu can wall jump in Brawl he will not be lowered as a standard. And also, there is a whole lot of Pokemon already, who thinks that there should be a copy of a Pokemon?
5. Mewtwo is a pretty good idea but there are some flaws in his idea. In general you know he is a legendary right? But the standards set for a legendary is much higher, thanks to Brawl's
http://www.mariowiki.com/Spear_Pillar. Leaving you expecting a legendary to be as powerful as Dialga or Palkia. And if a "legendary" can get defeated by a mere Pikachu or Jigglypuff alone then you have to wonder about the whole legendary idea. Making them all supposedly more powerful then a normal Pokemon would do no good to be a real character. If Mewtwo was to return though, it would help Pokemon a lot to make him a boss. Being more powerful then the ordinary Pokemon (like Rayquaza) would help his reputation.