I'd lead up to the ending in the pre-war phase of the Romulan war, where you know it's coming, there are regular engagements, but it's not a full-blown declared war yet. UESPA is rapidly expanding Earth's fleet, and suddenly everyone aboard Archer's ship is a terribly valuable commodity (heavily experienced officers) in line for promotion in an organization starving for senior leadership.
I love the idea that the ending is simply the senior officers being split up, rather than anything more dramatic or final. I think I would have the war already declared, because if ENT runs for seven seasons it would run from 2151-2158 in-universe, and that means the war has begun during the latter two and a half seasons. Maybe we get a chance for the
Enterprise to not be conspicuously present for every monumental event in history, and the war starts while the
Enterprise is still running diplomatic missions and they have little input on the opening battles.
Looking at films: when I watched the Kelvin films, I thought that they were made with a mindset similar to ENT but also as a reaction to ENT. Like ENT, the Kelvin films are a return to the "classic Trek era" (of course ENT goes much further back than young Kirk and Spock.) There's a decent number of references in the Kelvin films to ENT, so much so that Idris Elba's character is implied to have been present offscreen during the Xindi crisis. I think you can retool the existing Kelvin trilogy into a bridging trilogy between ENT and TOS.
First would be a film set in 2160 during the end of the Romulan War. Basically this is the extended finale for ENT after we split the crew up at the end of the series. We see all of the senior officers in their new roles: Archer has become an admiral, Shran is leading MACOs on the front lines, T'Pol and Hoshi are researching the Romulans, Trip is using his engineering prowess in RnD, etc. Eric Bana as Nero can be retooled as a prolific Romulan commander, no time travel shenanigans here. I'd also like to see Brian Thompson as Admiral Valdore reappear.
The goal of the Romulans has always been "reunification," which I think is code for conquest and subjugation of the Vulcan people. We're also told that they have a fascination with Humanity, and the war is primarily the Earth-Romulan War, yet Spock tells us in Balance of Terror that neither side took prisoners of war. We need to maintain continuity with Balance of Terror, but I think the fascination with Humanity needs to be expounded upon: perhaps Nero is breaking rank and attempting to capture Humans, or else masquerade as a Vulcan and sew discord.
The war should also tie back to a plot point in ENT: Trellium-D is highly toxic to Vulcanoid physiology, and if we place Romulus beyond the Delphic Expanse as we have discussed earlier, then this could come up as both a reason why the Romulans don't take prisoners off of trellium-coated starships and as a weapon against Vulcan-manned vessels. Instead of blowing up a planet like in the 2009 film, what if Nero is going to trellium-bomb Vulcan in an attempt to rid the population of logic and "liberate" them to the Romulan mindset of highly-charged emotions?
According to my FASA booklet, Operation Golden Pheasant is one of the last moves by Starfleet during the Earth-Romulan War. Memory Alpha tells me that the final battle takes place in orbit of Cheron, where the black-and-white-faced people from TOS come from. Cheron could be a world rich in trellium, where races enslaved by the Romulans (maybe Aenar again) are manufacturing the weapons for Nero's final solution. T'Pol is called to the front lines because of her experience with trellium; Archer is present at the battle, perhaps having reappropriated the NX-01 from its current captain as his flagship. Everyone reunites at the final battle, in 2000s blockbuster style.
I think Trip should be the one who is sent (or defies commands) to stop deployment of the trellium weapons. Maybe he recognizes that Archer and T'Pol will be instrumental in founding the coming Federation, and neither can be allowed to die at this battle. I'm perfectly comfortable with Trip learning here that Romulans and Vulcans are one and the same and engaging with them in close quarters, because he's not coming back from this mission. This is his Spock moment in Wrath of Khan. When the Romulan fleet is neutralized and the trellium is rendered inert or used against the Romulans themselves, Valdore and Archer negotiate the peace treaty via subspace radio and the film's epilogue skips ahead to six months later when Archer gives his speech at the founding of the Federation.
If you want, you can even have Riker and Troi cameo as they did in TATV, attending the conference via holodeck instead of making them the focus of the episode.
In my mind, this would resolve several of the problems presented by ENT's ending and avoids the Kelvin trilogy by tying its concepts directly into the prime timeline. It gives Trip a worthy death, ties together plot points from early seasons, and wraps a bow on the era.
