Only way a movie would be done is with Whedon but I think if he ever decided to go bigscreen with that verse then a joint BUFFY/ANGEL/SPIKE movie would be needed. Only question what would it be called
I guess BTVS is the more common known show.
The universe is referred to by fandom as the Buffyverse, and BTVS is seen as the flagship of the franchise (which is kind of annoying, as I think Angel
was the superior show). It has more name recognition. You throw out an "Angel" or a "Spike" movie and the average Joe will be like "Huh?"