I still need Krystal.
Call it hope beyond hope but I think it is possible other characters (without SSE missions) could be unlocked as playable characters through game save data from other games or extreme achievements. For years to come though, diehards like me will probably still be spouting such theories but in the mean time there is some validity albeit a stretch. It's possible we could see characters from the previous games not announced in this one announced as unlockables in much the same respect past stages are unlockable, though as time passes from the game's release in Japan, the less likely these theories become.
If this is foolish hope we can at least hope for a second Smash Brothers for the Wii just as there will be a second Galaxy, though waiting for the one next gen is fine too. Either way, this game has devoted a large part of development to exploring and refining new options for gameplay, with the ground work set in place by Brawl the next game will have more room for the addition of new characters most likely beyond an increase of just 10.