Pichu has been practically replaced by Sonic gameplay wise (fastest, not a heavy hitter per se, hard to control, etc.). Not to mention he was a clone.
Ike has replaced Marth and Roy. Not to mention, Marth in Melee was technically Ike in a different outfit/name.
Dr. Mario is doubtful, especially with how detailed some of the alternate costumes are becoming, not to mention he was another clone.
Young Link is up for debate. He was a clone, though now Link is based on Twilight Princess, so its possible Young Link would be a Wind Waker/Minish Cap/Phantom Hourglass style and have a unique moveset based on those games' items. Actually... Thinking about that just now, I actually hope they do that, a bit.
Edit: Forgot to vote for Falco and Ness in the poll. Falco is a clone, though it is possible he may get his own moveset, and possibly incorporate the rest of Fox's squadron as playable characters (though Sakurai'd have an angry mob at his door if he put in Skippy). Lucas seems to have Ness' ability, and was mentioned to possibly surpass Ness. So Ness is also up in the air, at least until he's confirmed or we learn more on Lucas.