Exactly. Natural borders would just make life unnecessarily difficult for modders.
But surely if someone is adding an
entire province (as mentioned in your earlier post), altering the mountains would be the least of their worries?
Anyway, I'd have thought that for the sake of practicality, significant new landscapes would be best added as new worldspaces reached by travel services (boat, carriage, mage travel etc.) or possibly via a dungeon crawl through the mountains.
Are there many mods for Oblivion that simply extend the borders in order to add new landmass?