Here's hoping for a real Blasto the Hanar Spectre movie.
In all seriousness, I think they should do a prequel. Shepard has a few different options for his background in ME, including the Akuze massacre and the Skylian Blitz. Focus on one of those and that way you would also be able to show Saren finding Sovereign and being indoctrinated at the same time. The movie could even end with the Council discussing the possibility of allowing a human spectre candidate.
In all seriousness, I think they should do a prequel. Shepard has a few different options for his background in ME, including the Akuze massacre and the Skylian Blitz. Focus on one of those and that way you would also be able to show Saren finding Sovereign and being indoctrinated at the same time. The movie could even end with the Council discussing the possibility of allowing a human spectre candidate.