A Battle in Trek I Would've Loved to See

[Nealithi]

Eh. The Cybermen vs. Daleks only had one super scene.
One million Cybermen against three Daleks. And the Daleks comment.
"Cybermen superior in one area. Dying."

It was a curbstomp.
Which with proper Borg is what a battle between them and the Dominion would be. With the Borg being the Daleks in this instance.
As for a movie with enough budget. Note the review for First Contact. They had to limit the Borg one whole cube again to keep it cheap enough for the rest of the movie. Which again goes against the original line about they will not come piece meal. I think you would need two or three movies to really do it well and the first movie better be awesome to bring them back for part 2 & 3.

[TGLS]

Is there any reason to believe the Founders can even be assimilated?

If the Founders can't be assimilated, the trick the Dominion would use would be to beam a Founder over to the cube to blow it up. The Borg don't actually seem to have personal weapons besides assimilation after all. Of course, then the Borg would just blowing up cubes when Founders beam aboard, which would probably stop the Dominion from doing that.

Hm. Would assimilated Jem'Hadar need ketracel white?