The problem is the budget, could someone do the Phoenix Saga on a TV budget?
I actually think you can do the Phoenix Saga in 2 films, one to set up the Phoenix as a hero and have her fight D'Ken and the Sh'iar Empire, the other film is where she becomes corrupted and becomes the Dark Phoenix. I do not think 3 films are needed (I think it would be hard to get 3 movies with 3 acts and a climax out that arc), but you need more than one.