Well, there's the Warp Jumper for a hard AI, and the preservation wardens for a minor faction (they're pretty light at first, but could get pretty tough if you get like 200-300 extractors, of course then you have the resources to fight them). There's also a new easy and a new moderate AI but I don't think I broke them as much balance wise, though the moderate one (Neinzul Youngster) is likely to launch 800+ ship waves at you on double frequency. So if you aren't used to handling youngling ships it could be interesting
The roaming enclave minor faction is also capable of making things interesting, but it's more of a balanced one because some of them ally with you.