Well looking at the sequel... I'm suspecting they'll follow the Batman Begins/Dark Knight Model. The first film reintroduced the franchise. The second one has the iconic baddie. For Batman that was the Joker.
I'm guessing for the next Trek that means we can expect to see Sybok and maybe V'Ger...
OK, maybe not them. I would bet we are looking at Khan. You really want someone that casual fans can quickly latch onto. Of course adding Khan could play havoc with continuity. There is no way he's a world leader in the 1990s in any sequel film. I wonder if they'd go with an Indian-American actor to play him.
If not Khan I'd bet on either the Borg or some major Klingon conflict. The problem with the Klingons is you'd need a face and I don't think any individual Klingon, aside from Worf, immediately stiands out in public perception.