There's a debate about that.
Apparently, when they were developing Super Smash Bros. Melee, they were going to replace Ness with the star of Mother 3 (or, as it would have been known here in the states, Earthbound 2). However, the massive delays on THAT game allowed Ness to stay in.
Now that Mother 3 is out on GBA in Japan, Ness might be retiring in favor of whatever the main character of that game's name is.
EDIT: Also, I'm thinking that Marth and/or Roy may be replaced by one of the newer Fire Emblem lords. Marth may be safe due to him being the first, but Roy was just put into Melee due to the fact that they wanted to plug his upcoming Fire Emblem game.