Roy, without a doubt, has the best chance. Arguably the most missed Melee absentee (tied with Mewtwo), as well as most advanced and tested clone codeset.
But DS represents the ability to bring a new character into the game. I think she's tied with Doc for second. It helps that DS has a good coder working on her, something that Pichu, Blood Falcon, and most other characters lack. I think Young Link will end up being good once that thread decides on an approach to take with him.
The biggest issues with making any of the clones legal is first, getting the clone engine to work. And since most of us have no idea how to code...We just have to hope this happens, lol.
The second issue is balancing them against 42+ characters (I think that's right...plus other possible clones). That in itself is a gigantic task, especially when we don't have the 42 already existing characters balanced enough yet. And playtesting and suggesting new stuff is the only way most of us will be able to help with this.