I love playing sieges battles defending fortresses against 3 Brutal A.I is what I live for! However, on siege maps there are a lot of units that you can't get and faction upgrades that you can't use... For example, Minas Tirith literally means tower guard however you can't any of those on this map because you need the white tower upgrade from the fortress (I think you understand what I mean). I don't know anything about programming and I understand that it isn't easy guys, don't think I'm hating or anything I just hope you guys find a way to make this happen so we can enjoy playing each map at its full potential.
This is also applied for other factions: you can't get Morgul orcs, vanguard, or Shelob (don't really care about her lol, others might though) on Minas Morgul.
I also tried the newly updated Misty Mountains and I loved the changes however there are also stuff you can't get because of the Gundabad spire upgrade on the fortress (like the new war bats, imagine having them hovering all over Dol Guldur!)
Of course there may be other stuff that I missed but I hope you understand the concept of what I'm saying maybe it'll help with the plans for the new factions that are currently being developed as well.