How about three kinds:
- Ridgy a la everything 1979-onwards
- Smooth a la TOS and late ENT
- Chang very-subtle-ridges (hybrid of the above?)
And wait, we don't even know if Klingons are gonna be IN this!
I thought the ENT explanation was fantastic -- couple of rogue eugenics augments unfrozen by Soong slaughter an entire shipful of Klingons, after which the Klingons understandably decide they could use their own augments, using Khan-type DNA as a base, disaster results. Humans pop in and save them from having to exterminate half their species, it's all hushed up so Klingons are left with grudge but no real motivation to exterminate everyone on Earth. And a lot of 'em are stuck with smooth foreheads. I thought it was pretty neatly done, and an entertaining story to boot.