I'm sure there are countless more species that played a minor part in the show (either via mention or sight) that are considered on par to other major factions of the Alpha Quadrant. Granted the Son'a aren't exactly a threat, but both the Tzenkethi and Sheliak were feared somewhat.
Would be nice to have some encounters with some of these guys instead of 'alien' designs. I was watching a DS9 episode yesterday (where Odo kills a Founder); the Tzenkethi were mentioned and it was hinted that they had fangs (thus they're either Cat-like or Lizard-like and not the traditional humanoid). The Sheliak (if I'm not mistaken) are crystal-based. Could there be some history between them and the Tholian?
I can't think of other powerful (but hardly seen) races right now. But still, how about it?