Believe it or not, Retro. Moreso if they decide to include him in DKCR3, but Smash is a different beast from DKC in a number of ways that would work in Rool's favor when combined with Retro's mindset.
--Smash isn't "Miyamoto's baby, so don't mess it up!", so there should be less creative restriction with regard to "what point in DKC's legacy to draw from"
--In contrast to a platforming game where it's tempting to make "fresh, new creative" characters, Smash is an all-star fighter, therefore those creative impulses would be spent on existing characters, not original ones
--Retro is a Western company, and thus would be more receptive toward Western franchises, or at least wouldn't flat-out ignore or Flanderize them
--Retro is very aware and receptive to most, if not all fan requests in a way that few companies are
The fly in the ointment is that Retro might prefer to draw exclusively content from their own games, not ones with K. Rool, but considering how big of fans they are of the originals, I don't think that'd happen (and it wouldn't matter if K. Rool starred in DKCR3 to boot).