A teaser you did not see comming (RAR!)

[Khaat]

Does it need another faction? Isn't there enough on the table already?

If the Imperial captain is "I got into the Navy to secure the Empire (chase pirates and outlaws), not burn citizens to dust!", and the Starfleet captain is "I saw the horrors of war first-hand and I'm not giving them another chance!" there's already plenty of internal tension.

[PREDATOR490]

If you were going to do a mini-movie then I can really only see the best way being a re-do of B5 human / Minbari war. The Minbari were completely dominating without pause but Sheridan did get at least one minor victory and was more of a character piece anyway.
That said, this was mainly due to the movie being a filling in of the blanks established by the series.

I expect a free form movie with ST vs SW would naturally be pumped up without any need to stick to the lore. Thus we have the rule of cool dictating that a single torpedo from the big E will destroy another Death Star remake.

Honestly, I would rather not see this film be made. This stunt was tried twice with Aliens vs Predator and both times it was utter garbage that satisfied noone.

[biostem]

It could be interesting to have the start of the movie focus on a Federation officer that somehow found him or herself stranded in the SW galaxy, and they're just trying to survive - maybe they've been stuck for a while, and have been able to adapt and learn about the technology/culture. Now fast forward a bit, and said person learns that what brought them to the SW galaxy is being attempted again, and they have to take advantage of the opportunity to return. Have things culminate in this person stealing a smaller ship, using the anomaly, and warning the Feds of the impending "visit" of the AQ by SW forces.

[Khaat]

It could be interesting to have the start of the movie focus on a Federation officer that somehow found him or herself stranded in the SW galaxy, and they're just trying to survive - maybe they've been stuck for a while, and have been able to adapt and learn about the technology/culture. Now fast forward a bit, and said person learns that what brought them to the SW galaxy is being attempted again, and they have to take advantage of the opportunity to return. Have things culminate in this person stealing a smaller ship, using the anomaly, and warning the Feds of the impending "visit" of the AQ by SW forces.

While your idea does have the Trek flavor, to boldly go and all, it's hardly a Wars-specific crossover - it may as well be any more advanced-tech galactic expanse setting the Trek crew explore. And other than the wild assumption that enough Wars-tech has been gleaned to give the AQ any kind of chance at a stand-up fight, where's the "vs"?

You'd be better off with "lost Feddies turn merc in the Outer Reaches, get in bad with a (singular) Hutt, Hutt invades the AQ with his/her syndicate/merc forces." The AQ could stand a chance, then. Hardly iconic SWvST, though, more "ST bad guy of the week."

[The Romulan Republic]

For an alternate universe story that won't effect established canon, I'd go all out grand epic, bringing back all the classic characters (probably animated to avoid recasting issues) or a "next generation" story with some of the old heroes in supporting roles.

For a smaller scale story to fit in existing continuity, I still prefer the lone ship in the Trek galaxy idea.

[U.P. Cinnabar]

What would be your response?

You mean, aside from:

Oh, God.

This is something best left to the realm of fanworks, as a movie treatment of this would break canon for both universes, and will only make the jihadists in both fandoms more fanatical in their belief that theirs is the One True Religion, and both religions will see any such movie as a betrayal of their faith by the soulless Hollywood machine, as as mentioned upthread, a good story would involve neither side emerging as the clear winner, and even a mediocre story would be so mired in fanservice, that, again, neither side would prevail over the other.

That's my two florins on the subject.