I'm far from convinced that this could work. The Stargate made-for-TV features are just as bad as the Babylon 5 ones, despite having a well-conceived mythos full of powerful characters and concepts. Just adding a feature budget for better sets and effects did nothing for them - because the hook in all this "mythos" business was that it was built piece by piece, episode by episode, in a great arc that obviously was not preplanned or even particularly well thought out, but was all the more consistent and innovative for it. Every revelation in a new episode was calculated to outdo the previous one, to throw a curveball, or to tingle a nerve tuned to an already familiar character or concept not seen for a long time.
None of this would be possible in a feature film, which has to offer it all in a nice package. There's nothing epic about such packages; epics call for worlds that the audience sees for the very first time in the feature at hand (regardless of whether these are all-new fiction or an interpretation of historical fact or preexisting mythology).
If Trek needed a war movie, the war should have been introduced in the movie. With a long and epic backstory, of course - but one never actually offered to the audience before this.
I mean, has a "conclusion" movie worked, like, ever