I am guessing that Loki will play a pivotal role in Avengers: Doomsday. Not only because of Tom Hiddleston's popularity in the role, but also because of where we last saw the character. Loki was placed in charge of keeping all of the timelines intact, and I'm betting that that will play a factor in whatever multiversal shenanigans Doom is planning. Who knows - with the movie delayed we now have a lot more time to speculate.