There was an episode of Justice League Unlimited (Epilogue) that was intended to be a series finale for JLU (although the JLU was renewed for a final season). But it was 99% a Batman Beyond episode and written as a finale for the entire animated universe of interconnected shows, with only a few minor connections, mostly via flashback, to the JLU series. The prior episode (Divided We Fall) was the proper series finale for the JLU (even though, again, they got another season to fight against the Legion of Doom).
So, I would take a page from that... admittedly, Epilogue aired two months after TATV, so I would have to look to the future to do so. Have a "Star Trek" Epilogue of 45-minutes air after the (actual) series finale of Enterprise.
The Series Finale
This would be another trilogy, removing the Terra Prime episodes and probably Bound from airing. Something akin to 9/11 or the Xindi Incident, i.e. an unprovoked attack that would lead to the Romulan War. But probably, since they're Romulans, have it blamed on the Vulcans (anti-Syrranite rebels) and maybe throw in some Orions (to develop them in place of Bound) and prop up the anti-alien resentment on Earth (in place of Terra Prime... although their might be a Terra Prime faction in the episode). Earth and Vulcan cut off diplomatic ties due to the growing unrest on Vulcan, and T'Pol is treated with hostility after Enterprise returns to Earth and are preparing for a more codified five-year mission. Archer and crew have to fight to keep T'Pol in Starfleet and Admiral Gardner is pressured to reassign the entire mission to Columbia and maybe just have Enterprise as a museum ship. Archer gives a speech that ends racism, while Reed and, I guess, Mayweather (and/or Sato) uncover the Vulcan extremists are actually Romulans in disguise ("they must be using some sort of undetectable surgery"), and this information is presented to the UE Council. War is eventually declared as Enterprise leads a fleet of Tellarites / Andorians / Vulcans / Rigelians? and Earthers into battle. Maybe they can figure out that Romulans are Vulcanoid, but decide to keep that info to themselves. No one important dies (save that for the movies that will never happen).
Epilogue
We open on the 31st century... Yes, that 31st century. Daniels is guiding a troupe of diverse agents in their ongoing efforts to stop a front in the Temporal Cold War. The agents: Captain Archer (c. 2156), Captain Riker (c. 2382), Admiral Sulu (c. 2305), Colonel Kira (c. 2382), Commander Tuvok (c. 2382), all bicker with one another over the best course of action. Using recent sets (Nemesis, IAMD) and tons of Trials and Tribble-ations-esque footage, we can see them involved in many top episodes (Balance of Terror, The Best of Both Worlds, Sacrifice of Angels, Twisted, Broken Bow, maybe even Star Trek IV: The Voyage Home) from across the franchise. Turns out they're mobile holograms of the ancient figures, utilizing their unique perspective to help the 31st century Timefleet. We do a Future Guy reveal... maybe it's Archer or a Romulan, I don't care, and really not much else. It's a nostalgia-fest, but not necessarily the best episode. You can give us a quick update on the Romulan War (especially if Future Guy was a Romulan pushing things in his favor), and that might be where most of the regulars make their quick appearance, but it's mostly an Archer and his Time Crew episode.
It ends on a minced version of the Captain's Oath with ship flybys and a combination of archived voices and new voiceovers (when necessary):
Kirk (NCC-1701): Space,
Picard (NCC-1701-D): The final frontier
Sisko (Defiant): These are the voyages
Janeway (Voyager): of Starfleet
Shran (Kumari): Our continuing mission,
Worf (NCC-1701-E): To explore strange new worlds
Spock (NCC-1701-A): To seek out new life
Sulu (Excelsior): and new civilizations
Cochrane (Phoenix): To boldly go
Archer (NX-01): Where no man has gone before