Enterprise was going to have a different Vulcan
Star Trek: The Original Series fans remember T’Pau, the Vulcan leader, who presided over Mr. Spock’s wedding ceremony on the first episode of the second season, “Amok Time.” (The episode was written by Theodore Sturgeon, who created the character.) So when Star Trek: Enterprise was preparing for production, producers thought a younger version of the Vulcan would work perfectly for the series. So T’Pau was the name used in casting calls. In fact, at one time, Babylon 5 actress, Marjorie Monoghan was being considered for the role of Sub-Commander T’Pau.
But as Theodore Sturgeon owned the rights to the character he created, that raised legal issues for the producers. So they decided to go with a new character, T’Pol, although, at one time, the producers did think about making T’Pau T’Pol’s sister to unite the two women in history, though that was dropped as well. The Enterprise did end up using T’Pau as a character, though, as she was an extremist who demanded the government return to Surak’s teaching.
Enterprise decided against using an established character
Using an already established character might have also caused some issues with continuity. Though we didn’t see much of T’Pau, we did learn that she was the only person to turn down a seat on the Federation High Council, and although that wouldn’t have had to necessarily come up in Enterprise, it was part of her future that was already established. Using a character that is already created can be daunting as well, especially if there limitations placed upon how that character can be changed or utilized. Fortunately for Enterprise fans, producers decided to go with T’Pol instead which allowed them to create a new backstory and set up a new future as well.