There will be a DKC-esque HTFA game. (more likely to be spinoff, I think)
There will be a spinoff crossing over with Yoshi's Island.
It would look more likely as a fanmade Yoshi game with the Happy Tree Friends replacing the babies, but why not? There will be a spinoff crossing over with WarioWare.
Very unlikely and would be more like a HTF-themed WarioWare fangame, but hey, this series itself is one crazy Crossover.