It's been implied and assumed, but not explicitly stated, that captain and warchief options on attack and defense flow from their tribe/class. So facing guys from the Feral tribe you can probably expect Caragor packs and Caragor riders. Mystic tribe you can probably expect necromancy, curse and probably spiders. I don't know how they've mapped it out if at all, but it would seem a huge missed opportunity if all orc captains of class X got the same siege options regardless of other factors.
Also the whole time passing thing and dividing up Middle Earth into regions conflicts somewhat with a broader, RTS-style field of view for things like sieges and the movement of armies etc... It's not clear in SoW how you know about stuff happening outside of the region you're currently in. It was said in dev streams that "time only really passes in other regions once you travel to it." Essentially a lot of the automated parts of the Nemesis system are suppressed if you're not present to see it happening. Unoccupied regions stay in stasis so, for example, you don't come back to find half your captains and warchiefs dead or deposed. So I don't know how the game telling you that your fort in Nurn is under assault when you're in Gorgoroth would work. The way they've described it just traveling to that region defend the fort would be read as "time passing."
So I imagine you'll get a story mission to introduce the concept of defending your fort (possibly after you capture the very first one) and then it will be some sort of check that, while you're in a region with a captured fort, the game may spontaneously generate a fort assault for you to defend against. And if you don't respond, you lose the fort, the region and probably all your orcs at the fort.
I'd think that simulating the Nemesis system in non-active regions starts to become too much overhead for them when you're simulating......300? orcs and armies moving around, leveling up, dying, getting new traits, so and so forth. Starts to sound a bit like DF at that point.
As an aside, they did say that enemy orcs you've met in other regions will hunt you across regions. Which is pretty cool.