Having expansions on the Cardassians, Breen and Tholians will definitely be appreciated to expand the range of antagonists.
Are the Nyberite Alliance based in the Alpha Quadrant? They’d be interesting to see as its implied to be open to outside races being members of their military. Could it be a Foreign Legion type ideal there or is the Alliance multi-species and open to others similarly to the Federation?
And also the Tamarians as well (the ones in a TNG who had a brief war with Starfleet and an officer adopted a human boy) just to get a feel for that conflict as well.
More Federation races will be good to see as well. In general it kind of feels like the Beta Quadrant saw a lot of the Federations 23rd Century history and the Alpha Quadrant sees more of its 24th Century history, at least to me.