I think lifting characters from VIII is really hard. My first thought was Seifer, but he'd play too much like Squall. Same comparison can be made with Edea and Ultimecia. I think maybe you could lift Laguna as the hero and Seifer as the villian, maybe change Seifer's use of the gunblade that favours power over speed to kind of differentiate between the two.
For me, the easiest choice has to be Tifa. For one there's not that many female characters in there, she's one of the most iconic female characters in the series and as she uses her fists it would make an different playstyle and diversify the roster up a bit. And if it's not going to be her, then it's either Sabin from FFVI or Zell from FFVIII. I can only predict they are going for one more hero and villian from each game, but it's hard to think how that can apply in certain cases. What other notable villian is there in FFVI apart from Kefka for example?