I think four DK reps isn't an unreasonable thing to ask, considering that it's one of the franchises that led to Nintendo's success, it's still pretty big, and both Dixie and K. Rool are well-known characters. Dixie's recent confirmation in Tropical Freeze makes me think she's more likely, in the case we only get one new character. While it's true that the character roster was finalized a long time ago, I think it's possible that Nintendo either told him in advance about Tropical Freeze, or at least told him to add Dixie Kong to Smash.
I'm kind of hoping that Diddy and Dixie get paired, actually. As long as they switch with a taunt (Diddy shouldn't lose any of his moves), it would be neat to have a tag team. That being said, Dixie would still be fun to play as her own character!
If there's one thing I think Dixie SHOULD have, it's the ability to glide. Also, bananas would be nice, but if she's paired with Diddy, it's not that important.