I remember during pre-Brawl, Dixie didn't have nearly as much support as she does now ( Krystal actually had much more than her...funny how things have changed). I think it was because Diddy was the obvious next choice, and then she and K. Rool were fighting for that 3rd DK spot, and it was a spot that was most likely not gonna exist anyway. But now, it's a spot that is way overdue, as is there for Dixie to claim.
To me she is one of the most important Nintendo females...probably 4th behind only Samus, Peach, and Zelda. So now that we already have 11 female reps confirmed, it would simply be an injustice if Dixie is not playable. I remember watching my brother play DKC on his gameboy when I was only single-digits, and was always interested in Dixie Kong. She's an iconic character. She fronted a major Nintendo game, and along with Samus (and Peach, I guess) is the only female to do so.
I would be happy if both she and King K. Rool were playable, but to me she is far more likely and deserving. People just know Dixie Kong...in truth, I didn't even remember K. Rool until I joined Smashboards and saw his support. He unfortunately hasn't been relevant in many years - Dixie is in TF, and there's no way Sakurai wasn't aware of that during production. Plus, he had already planned for Dixie in Brawl so we know she is on his mind. On top of this, she'd be incredibly easy to design a model for, and has an easy moveset to come up with as well.
If there are more characters unlockable (which I do feel like there will be), Dixie should very well be one of them. Dixie fans has hope, which is more that 95% of the threads here have right now. Go Dixie Kong!