As much as I want playable Breen, I see them more in a Dominion faction.
Kazon, yeah. If the Romulans get Hirogen, Klingons should get Delta Quadrant races, too. I'd find the Hirogen much better for Klingons, but then I don't really care about Romulans and their faction in general. Kazon would at least "fit in" because they were a cheap Klingon replacement in Voyager, and would add a few more fancy hairstyles in the Klingon customization. Not that I would play one, but I would use their hair for my Nausicaans.
What would they possibly have (economically, technologically, culturally) that would make THEM interesting for the Klingons?
I mean they have trashcans with warpdrive, they produce more toxic waste than Enterprise-era Klingons...did I miss anything of importance?
Of the various races in the TNG and post-TNG era I'd say the Chalnoth would be nice.
Would be cool, but probably not going to happen due to some licensing problems.
For the same reasons the Kzinti from Star Fleet Battles were renamed to Mirak in Starfleet Command, although just about everything else about their ships and history is identical to their SFB counterparts.
Breen conquered? The Empire tried, and the ships never came back. Seeing what fancy stuff the Dominion gave the Breen, it can be assumed they also gave them technology. Unlikely anyone could simply conquer them while fighting another war.
Calnoth, I'm for it. I'm for any race with a good set of tusks. They should also be easy to conquer. They have no government anyways.
What Malon can offer? You said it: waste! That makes great biological and chemical weapons!