Berk's Forumvine

According to IMDB, Drago is going to be in the movie. I assume that he and Grimmel will wind up being allies in some capacity (possibly only temporarily), if only to keep the plot from getting overly complicated.

It does raise the question of what their relationship would be, though. One would initially think that Grimmel would be Drago's superior, but I actually think it would be more interesting if Grimmel was an underling who managed to become the main threat. Perhaps his obsession with hunting the last of the Night Furies might cause him to attack Hiccup and company before Drago had assembled his forces.* That would be a great little subversion of narrative conventions.

* A tangent: it's worth pointing out here that, if I recall correctly, we never see what happens to Drago's army after the battle on Valka's island. Those troops could easily form the basis of a new army for Drago, or a force for Grimmel to take control of in his leader's absence.