Definitely the female Romulan commander was a highly compelling character. And her real-life lookalike daughter could have kept the legacy going in film.
The female Commander could very well have suffered massive repurcussions from Kirk's actions, giving her all sorts of motivations to pursue revenge. If she had a Bird of Prey, furthermore, you basically could have had a TWoK-tier space battle.
I would have preferred Kor, Khang, etc... to become recurring characters in a hypothetical fourth season of TOS. And for those saying Gary Mitchell should be the film villain, the logic is sound. But I think the best use of Mitchell would be as the final adversary of the TOS TV series. Bookending and all that. That's something that Star Trek Continues conceptually got right.
But I disagree somewhat with the execution of ST:C. Instead of being a villian, what if Gary Mitchell tried to be a hero? Tried to redeem himself and humanity, using his godlike powers. But in a case of "the uncertain business of doing good," Mitchell really screws it up.
Maybe Mitchell takes a cue from the Organians, and tries to "improve" planets as the Klingons and Federation were challenged to do. But he actually causes tremendous and spectacular damage. His application of incredible energies to problems is not the solution. So Kirk and co. have to stop him, maybe enlisting the help of other godlike figures from TOS.
Ultimately, it's more tragic than TWoK, because Mitchell is trying to make amends. But he thinks too much like a "big man," not co-operatively. So he fails. And Kirk reluctantly has to put him down, one more time. Maybe after Kirk shows him the error of his ways, he commits suicide before anyone can stop him.
This would be a good ending arc for TOS Season 4. The female Romulan commander would also have been a good recurring character, if not in a TOS movie. I think that a Mitchell story is better suited to television.