The Romulan War with Earth is one of the biggest storylines never realized.
When Star Trek introduced the Romulans in the 60s, the big reveal was that they looked just like Vulcans. A fact that even Spock didn’t know about. The Romulans were a society shrouded in secrecy and that was something demonstrated in the original series episode “Balance of Terror”. What made this even more shocking was that Earth and the future Federation had already gone to war with the Romulans.
Due to the tech at the time, the forces united forces didn’t have the ability to see what the Vulcans looked like. That was until the first season episode of the original series.
Seeing as how Star Trek: Enterprise was a prequel series to the original series, it set up the potential of finally seeing the Romulan vs. Earth war play out on screen. That was all set up to be done in the fifth season and beyond for Enterprise.
Then the series got canceled.
Star Trek: Enterprise’s slow-burn approach hindered it
Back to the Future is the only property that ever got time travel. Enterprise thought that it’d be a great idea for their show. That the Temporal Cold War could be their hook. It wasn’t. The show turned many off in the first season as the series hung on this idea of the Suliban being agents of some distant-future mastermind (that would end up being Jonathan Archer all along).
It wasn’t a bad storyline, exactly but it was the worst of the series. With other ones including the Xindii, their trek to the Mirror Universe, dealing with the Vulcan, and the formation of the Federation all being parts of the show, the Temporal Cold War just didn’t fit.
The show was supposed to tell the story of the Federation and how it was built but instead, its early episodes were about a nomadic group of aliens with a skin condition. It truly hurt the show’s potential.