Some others I think have a decent chance:
Dixie Kong:
+Was planned to be playable in Brawl in some fashion.
+Covers the relevancy issues Rool has.
+Would satisfy most fans clamoring for more Donkey Kong representation.
-May not have a particularly unique moveset.
-Not especially popular.
Rayman:
+Most likely carries a lack of any licensing issues since his trophy is in the game.
+Unique moveset potential.
+Relevant, popular, etc.
+Has the least issues to being added among any potential third party candidates.
-May not have been seen as a good candidate since Nintendo only used his trophy.
Captain Toad:
+The concept of a playable Toad has been a popular one for nearly 15 years.
+Distinguished from regular Toad.
-While popular, he's far from the most well known or wanted Mario character.
-Moveset implementation may be difficult.
Dixie Kong:
+Was planned to be playable in Brawl in some fashion.
+Covers the relevancy issues Rool has.
+Would satisfy most fans clamoring for more Donkey Kong representation.
-May not have a particularly unique moveset.
-Not especially popular.
Rayman:
+Most likely carries a lack of any licensing issues since his trophy is in the game.
+Unique moveset potential.
+Relevant, popular, etc.
+Has the least issues to being added among any potential third party candidates.
-May not have been seen as a good candidate since Nintendo only used his trophy.
Captain Toad:
+The concept of a playable Toad has been a popular one for nearly 15 years.
+Distinguished from regular Toad.
-While popular, he's far from the most well known or wanted Mario character.
-Moveset implementation may be difficult.