Star Trek: 10 Things You Didn't Know About T'Pol
3. She's Half-Romulan
Had Enterprise been approved for a fifth season, there was a major shock coming for the audience in terms of T'Pol's parentage - specifically, her father. Though he had been mentioned in the show, there was very little revealed about him on-screen. The fifth season would have revealed him to be a Romulan spy, operating on Vulcan.
This was actually a re-use of the original plan for Lt. Saavik, who first appears in Star Trek II: The Wrath of Khan, played by Kirstie Alley. She was immediately a more emotional Vulcan than the franchise had previously offered. This was due, though not stated on-screen, to the fact that she was a half-Romulan.
When the Search for Spock was released, with a re-cast Saavik returning, played now by Robin Curtis, this plot element was quietly swept away - with Curtis's performance matching the stereotypical Vulcan portrayals.
The return of the idea, for use with T'Pol, would have posed an interesting challenge for the writers. At this point in Star Trek's history, no human had seen a Romulan and lived to tell the tale. In Balance of Terror, Spock was unaware that the Romulans were related to the Vulcans. So, how were they to explain T'Pol's parentage, while keeping the canon intact? Alas, with season five a pipe dream, it seems like this question will never be answered.