I'd like for the trilogy to have a villain who has a compelling story arc that allows me to believe that their villainy is justified from their point of view. If they need three movies to do this villain justice, so be it. If they can make a new and compelling threat for each movie, so much the better. But don't make a new villain for each movie just to have a new baddie to fight, or make them all Force users. Bring back a Tarkin-esque character, or an entirely nonhuman threat for a change, such as an impending supernova or the death of a planet due to an asteroid collision. Yay variety!