I like what the novels have done with the Tzenkethi, a race really only mentioned in this episode, and it turns out that Starfleet was waging a border war with them as well at the same time as the Cardassians.

The novels definitely fleshed them (and the Breen for that matter) out really well (although right now all I remember is their ships had no unused spaces, consoles on the floor, walls, ceiling, kinda dizzying, and that an incident was engineered with them early in the occupation to help justify Cardassian buildup on Bajor)