Interesting idea. Each NPR could have a set number of systems beyond which it would not transit unexplored JPs, from 0 to unlimited but more likely in the 2-4 range. That might change given certain circumstances, such as being attacked or finding aliens in its home system. However, the NPR would transit jump gates.
Well, I specified system generation and not unexplored so the AI would be free to venture into systems generated by other NPRs or the PC for war and trade, but that idea is kinda neat in that two reasonably distant AI could have a prolonged border clash without one wiping out the other as they were too far away to bother with, just a contested system or two between them.
I'm mainly interested in players having the option to put the breaks on NPR exploration/expansion; I like long, slow games, and that means really cracking down on NPRs if I don't want them to lag out and die.
A middle ground between Pre-TN and Post-TN empires could also be interesting; Pre-FTL TN civilizations. Locked out of researching jump engine or jumpgate tech.
Spacefaring conventional empires would also add some nice flavour. Conventional engines, maybe some primitive weapons and sensors, luxury transports instead of cryo, etc.