This has been discussed elsewhere, but I believe it warrants its own topic.
So in concept this is a simple thing, something I believe would greatly benefit the immersion factor of the game would be for a number of enemy gangs during the campaign are tracked by the game so that they are a persistent foe. One that grows, that you can cause lasting harm to, that tries to take your territory(assuming that's an aspect of the game) and can steal your stuff and use it. Much like your own gang, there should be enemies that decisions, losses, wounds and deaths matter in the long term, in large part because it crafts a personal narrative for each player.
As for how it should work, the basic aspect is the same sort of randomly generated gang should happen, except it is saved and tracked much like your own gang.
Now that leads to other questions, some of which I have no answers to. Such as how hard that is to do, how many gangs could potentially be tracked, and how it could track enemy gang progress outside of its battles with you.
My answer, without knowing any of the technical requirements, is that there could be a mixed model. Each of the other Houses should have at least 1 persistent gang, but to me having only 6 gangs active in the Underhive at a time sounds a bit underpopulated, therefore they could also have randomly generated gangs for single scenarios that you would also square off with. That way you do have interesting rivals, but there's also a sense of scale, with random gangs passing through. Of course if it's possible the higher amount of persistent gangs the better.
And of course if you ever diminish a gang to the point they can no longer fight, for example by killing them all, they're simply replaced by a new randomly generated gang.