It'd be obvious, if you can only make one movie it has to be about what's going to happen next in the storyline, we already have info on everything else and it'd be killer to see what happens now that winston did the recall and if there's gonna be another omnic crisis etc
Plot would be the resurrection of overwatch and the founding of blackwatch and/or talon. It would be in the same animation style as most of the lore videos and it would end on a clifhanger with talon looking like the good guys while overwatch is made to look bad again.