To paraphrase and misquote G.K. Chesterton, a Romulan movie hasn't been tried and found wanting. It hasn't been tried.
Let's see, Romulans in the movies:
Star Trek: The Motion Picture - No Romulans.
Star Trek: The Wrath of Khan - Romulan ale. No Romulans. I think the Klingon Neutral Zone in the movie was originally going to be Romulan, but the name was changed to be more recognisable, a bit like when in -
Star Trek: The Search for Spock - the Bird of Prey complete with cloaking device was going to be a Romulan ship and our pointy eared friends the bad guy of the movie, but Nimoy wanted Klingons. The Klingons were to have stolen the ship, but that plot detail wound up on the cutting room floor, so no Romulans in this movie either. Klingons 1, Romulans 0.
Star Trek: The Voyage Home - drives home how Klingon the Bird of Prey is. No Romulans.
Star Trek: The Final Frontier - Romulans! Or rather Caithlin Dar, a single Romulan. No head-ridges like TNG et al, but she does have TOS era pointy ears. When she, the Klingon and the Federation Ambassador are taken hostage, the Federation and the Klingons send ships to the rescue... but nobody even considers the possibility the Romulans might. Nobody takes our villanous elf-Romans seriously, it seems, and the first Romulan in the movies turns out to be a really, really minor character with no plot importance. Klingons 2, Romulans 0.
Star Trek: The Undiscovered Country - More Federation and Klingon angst as a conspiracy of some hardliners on both sides try to keep the cold war going. Plus a Romulan, Nanclus, a figure even more minor than Dar but who also wants to keep the Feds and the Klingons hostile for obvious reasons. A nice additional touch, but arguably even more ancilliary than Caithlin Dar. Nanclus, incidentally, is the last Romulan seen without those TNG ridges. Klingons 3, Romulans 0.
Star Trek: Generations - The ridges debut. On a corpse. The Romulans are briefly mentioned as a possible enemy, but it soon turns out they're a largely unseen injured party and the real enemy is... the Klingons. Again. Klingons 4, Romulans 0.
Star Trek: First Contact - No Romulans.
Star Trek: Insurrection - No Romulans.
Star Trek: Nemesis - It's a film where the Romulans are the bad guys! Only, actually the main bad guy is a human, a clone of Picard. And most of his henchmen are the never-heard-of before Remans. So where does that leave the Romulans? Why, some of them get to become corpses, and Dina Meyer's Donatra gets to join the heroes! Romulans and Romulan politics more important here than in any Trek movie, but ultimately it's more about the new things thrown into the Empire - a Picard clone and a gothic nosferatu race of vampires - then our elf-Roman friends. The one movie that has Romulans in any significant way and they're shafted to a role as minor as the Klingons in The Final Frontier... and this movie is a lot worse.
What does this all add up to? At the very most, the Romulans have been relatively inconsequential supporting players on the big screen, despite having to their credit being the bad guys of some of the best episodes in the franchise - including TOS's 'The Balance of Terror' and 'The Enterprise Incident'. The biggest they've ever been, in Nemesis, they're victims, briefly the duped allies of the big bad guy, and then the rather useless defectors to the hero's cause. I really doubt this upcoming movie will have Romulans, but I'm hoping one of the sequels might...