It has to be a war that results for a century in the Earth/Federation maintaining a "wall in space" separating the losers from the winners.I would not have minded a cold war on Enterprise with a few skirmishes, but not the actual war, which could've been done with a movie trilogy. DS9 did the war. No need to do another war on a weekly series.
IMHO it has to be a hot war, really a fairly large one too.
As far as the Dominion War on DS9, I'm a pretty hard core Trek fan, and I've never seen many of the war episodes, partly because DS9 became my least favorite series. By the time the Dominion War was center stage, a large block of Trek fans (and the general audience) had already walked away from the show.
So a "Earth/Romulan war" could have been depicted in a couple of seasons of Enterprise (or a new series) and not subject the viewers to something they've already seen, because most of them never saw the seasons of DS9 that showed the Dominion War.
