There's definitely not an obvious winner here. On one hand, Dixie seems more likely at first glance due to being popular, important, and having a prominent role in a very recent and successful Donkey Kong game. And frankly, I think that does make her quite likely as far as DLC characters go. On the other hand, K. Rool is even more popular, has more overt potential as a Smash character than Dixie, and while he hasn't appeared in a Donkey Kong game in awhile, his importance to the series as a whole is undeniable (his trophy in Smash even outright refers to him as the Bowser of the Donkey Kong series). K. Rool has always been one of the most popular Smash requests, and considering that the first DLC character was literally THE most popular Smash request, it bodes well for K. Rool.
At this point, with so little knowledge on how DLC characters are chosen, it's difficult to label one as more likely than the other. But personally, I think both have a fair chance, and at least one of them is likely going to make it in. Either way, both would be fantastic choices for the roster.