Killing off Trip Tucker (Connor Trinneer) in Shran (Jeffrey Combs), their Andorian ally. Trip sacrificed himself by rigging an explosion to kill Talla's abductors and save Captain Archer's life, but his death happened so late in the episode, there was little time to mourn Commander Tucker.
Trip's death in Star Trek: Enterprise's series finale is canon, but there is actually wriggle room to change or even reverse it. The events of Enterprise's finale as depicted was a holodeck program used by Commander William Riker (Jonathan Frakes) and Counselor Deanna Troi (Marina Sirtis) during the TNG's 24th-century era, the way his death played out in "These Are The Voyages..." may not be entirely accurate since it was a holodeck simulation. The details could have been wrong, and Trip may not have perished as Riker's fantasy depicted. As such, there are ways to resurrect Trip or change his fate by simply ignoring the Enterprise finale.
Why Enterprise Finale Killing Trip Was So Wrong
Among the many poor decisions surrounding Star Trek: Enterprise's series finale, Trip's death is one of the biggest. To mark the end of his 18-year run of Star Trek shows that started with TNG in 1987, executive producer Rick Berman decided to make the Enterprise series ender about Riker looking at Enterprise in the past. But both the audience and the Star Trek: Enterprise were outraged by the end result, which reduced the series' characters to non-entities in their own finale. After all, those weren't the real Captain Archer, Trip, T'Pol (Jolene Blalock), and the other Enterprise crew, in the episode, they were merely holograms in Riker's role-playing game.
Killing Trip in the manner they did only made Enterprise's series finale worse. True, he sacrificed his life to save his best friend, Captain Archer. But the time jump between the penultimate Star Trek: Enterprise episode, "Terra Prime," and "These Are The Voyages..." means Trip's story, and his romance with T'Pol, wasn't satisfactorily resolved. The 'real' Trip was last seen in "Terra Prime" and then the audience had to watch Tucker's abrupt death, which happened 6 years later. Killing Trip, a beloved Enterprise character, was played for a series finale shock value that backfired.
How Star Trek Could Easily Bring Back Trip Tucker
Given that the scenes in the series finale are just Riker's hologram, it's a simple matter for any of the current Star Trek shows to say that the details of Trip's death were incorrect and what Riker saw on the holodeck was not actually canon. Of course, Trip is long dead in Star Trek: The Next Generation's era, but he could have had a better demise, or he preferably retired and lived to a ripe old age after his pioneering career of space adventure. Trip could also be recognized as an even greater hero by Starfleet and given fitting honorariums like making the USS Tucker in Star Trek Online a canonical starship named after Trip.
But the best way to honor Trip would be for Strange New Worlds, set a century after Enterprise, could also recognize Trip, but Lower Decks and Prodigy could easily have him appear in animation with Connor Trinneer reprising his role. Any of these solutions would remedy the unfortunate way Star Trek: Enterprise killed Trip Tucker.