I've been thinking that Garth would be a good villain and present something we haven't really seen in a Trek film, Starfleet against Starfleet, when they are playing for keeps. We got to see a little bit of a conspiracy in TUC, but the main showdown was between the Enterprise and that BOP. I would love to see two Starfleet crews match wits with one another. I would love to see Kirk go against someone he admires, someone that might be a lot like him but made some tragic choices that turned him into a villain. A cautionary tale for Kirk, something to make him a little less puckish.
I think Garth could be a great Khan-level villain and using him wouldn't result in any major comparisons to the more iconic Khan and Ricardo Montalban. There's a lot you could do with Garth and a lot more different places you could go with him with less chance of screwing it up and dishonoring what came before.
Some other potential villians, the female Romulan commander from "The Enterprise Incident", M-5 computer, Janice Lester, or Finnegan. Bringing Mudd back might not be a bad thing either if they wanted a lighter movie. And there's Kor, Kang, and Koloth. If you want a villainous species, I would love to see the Tholians or the Gorn. I think the FX are there now to do both of them justice.
Of course they could just create a new villian whole cloth.