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.
I came up with some 'reason' why the Kazon would have been in contact with the Klingons, but that works for Hirogen as well. There was a Voyager episode where they met a Klingon ship, with a crew unaware of the war being long over. That says there were indeed Klingons in the Delta Quadrant. (Romulans, not that I saw any.) Its been a while since I watched, but if I remember correctly, said Klingon ship stayed in the Deltra Quadrant or was destroyed there, they didn't go home... Hm. Says any race there could have met the Klingons, or found the remains of the ship and searched for the origin.
And then you have General Kurok, Lib Borg Klingon, who took over a sphere. He got his Klingon identity back, AND Borg technology. So he could have returned, and brought some allies with him.
Actually, there is not a single reason why the Romulans would be allied with any Delta Quadrant species. Klingons have 2 possible ways from Voyager alone. So really, give the Romulans species from the Alpha & Beta Quadrant (Flaxians, hint, hint), and leave Hirogen to Klingons.