Hmmm...what next. While I wouldn't mind a Klingon- or Borg-centric movie, I wouldn't mind seeing some other things like the Kzinti that were going to make an appearance in the next season of Enterprise before it was cancelled.
Then there was the Kinshaya...this was a FASA construct, and the name would obviously need to be changed, but these creatures were said to have almost destroyed the Klingon Empire, coming from the far side of Klingon territory, opposite the Federation. The movie could still obviously have Klingons in it, but the true adversary would be the Kinshaya - for if the Klingons fall, the rest of the Alpha Quadrant would be easy pickings.
Gorn and/or Tholian appearances would also be good, but they are not as clearly known by non-fans as the Klingons or Borg would be. I'm thinking the new movie will either likely bring in a well-known adversary, or one that is almost completely new and unheard-of so they can paint on a fresh canvas.