You know would be great about a Deep Space 9 movie? (everything!)
The fact that it would be the opposite of these new "Star Trek" films. The new "Star Trek" films are written not for Star Trek fans, but for general movie going audiences. They have no substance, and are just lights and colors with zero character, development, or even science fiction.
A Deep Space 9 movie could be a real Star Trek fans movie: science fiction, political and social thought, character development, all the things that made DS9 the red-haired child of the franchise.
My idea would be to do the one thing they forgot to complete in the series: Bajor joining the Federation. I'd love that as a premise for a DS9 big screen adventure. Remember "The Circle" 3-parter that opened season 2? That trilogy was better than all the reboot movie and all the TNG films combined.