We could always only leave bosses and change levels requirements. However, I don't think a movie could be made out of the entire series, since each games won't follow each others necessarly. Plus they all have one playable character each, different between all games.
It's the same for the Reign Of Legends series, altough a movie might work better with these, because they have 3 vs 3 battles. You would need to cut out a lot of loading time, though, and most likely speed up some parts (especially if you do ROL3)
The Reign Of Legends 3 would be a bit Terminatorish, though.