On the contrary. Not ships and costumes belong in the C-Store. THAT belongs in the game for free. Races, C-Store, that works.
Calnoth would be my first choice.
From the Delta Quadrant, Hirogen seem to fit better in the KDF than a future Romulan faction. I could also see Vidiians (who admire Klingons and would be likely to surrender to them) and Voth (originate on Earth, might want their ancient homeworld back) in the KDF. Voth would also go along with the other reptilians in the KDF. Both Vidiians and Voth are not members of a faction that may be planned for the future.
I could also see Kazon join the KDF, for the sake of getting better hairstyles for the Klingons, lol
Kazon might side with the KDF to get better technology; they are already warriors and have some unfinished business with the Federation.