I completely disagree with you, DrH. The reasons are multiple and encompass various aspects. I will summarise them to make it all simpler.
1. The kind of system you proposed is objectively too ossified and complex to adapt to a Mod with these standards. We are not talking about some map features or exceptions as Arnor; this is an entire structured and canonical faction to deal with, characterised by recognisable distinctive and unique traits as all the other ones. Having different designs depending on selected maps would 'ruin' the very conceptual unity of the faction itself and complicate everything without any real need that justifies this. Why would you ever make things complicated when they are already quite fine-tuned at the moment?
2. The main reasoning underlying your proposal is, in my opinion, quite naïve, if we take into account the basic gameplay requirements of the game. It would undoubtedly be a reasonable argument if we were to view things in a very purely logical perspective; but I'm afraid it doesn't apply to the common and usual context. For example, I could also question: why does Lothlórien start with luxuriant forest-castles or forest-camps in hostile (in a geographical and physical point of view, especially) maps as Mordor or Mount Doom? Can't we have (I'm inventing) a burnt/marred design when it comes to this sort of environment? The same could be said about Rivendell's holy sanctuaries and Gondor's mighty fortresses. This logic is just too dry to be even slightly manageable in practice.
3. The central heart of the Misty Mountains is the Moria subfaction for prominent reasons: the known iconic reference to the now legendary Moria sequence of the Fellowship of the Ring, the ordinary Goblins stick exactly to PJ's early interpretation (which I think is absolutely superior to the one of the Hobbit trilogy, in spite of the respect I have for the latest trilogy, and an absolutely coherent transposition), the Balrog lies dormant in the deepest caverns of this lost realm and, although set in the Hobbit timeline, this subfaction is the closest of the three to the War of the Ring timeline that most of the factions follow. Nevertheless, the faction will anyway contain mostly Hobbit-related content and I thus hope more improvements will be pursued in this direction; but I guess the 'supremacy issue' among the three souls of the Misty Mountains is undeniably tied to Moria due to the points exposed above. Nothing would ever capture the entire dark, eerie, dreadful and ghoulish essence of the Goblins better than Moria's cinematographic rendition
4. Connecting to the third point, the graphical design of Moria was exactly fashioned on those canonical and iconic premises, recreating the ghostly 'tomb' about which Boromir talks (the very colour of the buildings has been darkened to resemble even more what we see in LOTR). The fact that this once prosperous Dwarven kingdom is now swarmed by packs of wicked Goblins is well displayed by the current representation and it couldn't honestly do more honour to this race's true nature.
5. I had the feeling that the Gundabad subfaction is portrayed more as the gathering of the Defiler's hunters, embodying thus a wandering motive different from the major characteristics I listed in the previous passages. I also don't like very much the vanilla design of BFME2, as it reminds me of how loose and flawed the Goblins were back in the day (with little consistency and even less adherence to the lore or to the films). Therefore, I sincerely believe, if we really were to choose the hegemonic subfaction out of the three, I would rather opt for the new, unique, innovative and very Edain-ish Moria
P.S. It's really interesting to notice how this faction manages to put me in an unusual good mood. Don't misunderstand me though, my awe is primarily due to firm contempt towards such nasty creatures; in addition, the fact that many grotesque sides of BOTFA will hopefully be explored properly is a favourable scope for hoping that other eerie aspects of the same film will be considered as well...