Clone characters aren't usually ever intended to be in the games from the start. Usually they are extras that were built from characters that were already created for the game. In Smash 64, Luigi & Ness were built from Mario, Captain Falcon was built from Samus, and Jigglypuff was built from Kirby. They were then placed as extra, unlockable characters in the game.
Clone characters are usually a result of generosity. Wolf was an example of a character who was highly requested and only made it in because he could easily be built from Fox and there wasn't time to make another fully unique newcomer. Wolf got "lucky" as Sakurai said, just like Dr. Mario, Dark Pit, and Lucina. The only clone newcomer you could argue was intended to appear from the get-go was Lucas in Brawl.
This is partially why I don't find Dixie Kong particularly likely under
normal circumstances. Rather we'd have to rely on Sakurai recognizing her popularity, and deciding to add her as an extra character built off of Diddy Kong if there is enough time. But, seeing as she would be vastly different from Diddy Kong, it would increase man hours much more than Dr. Mario, Dark Pit, and Lucina would.
That being said. There are three clones I think could happen:
- Pichu, because of requests.
- Alph, because people were upset that he wasn't seperate.
- Impa, because she is requested and might not work well as a Sheik costume.