Far as I can tell, modders have a means of replacing the map. I've heard there's an entire map for the Inner Sphere out there.
With Flashpoints, it's possible to construct branching paths which use decision points (or potentially, if possible, failure states of missions) to branch off.
Since it's confirmed there's no technical limit to the number of missions, it's possible to do as many as the baked-in story campaign (which is roughly 15 missions if I count right).
There's also Flashpoints appearing randomly but once finished, aren't available anymore, but if they time out they'll just potentially reappear again.
We do know, however, there aren't currently ways to build maps from scratch so there's that limitation.
But give the modders time, and a lead, and custom Flashpoints might actually be capable of being campaigns of their own. Given enough people doing them? It's entirely possible from these points I've listed to have a mod where there are literally a "deck" of full-sized campaigns you can steer your poor mercenary outfit into fortune and glory across the years between the Third and Fourth Succession Wars.