I had originally wanted to post a proper proposal, with a proof-of-concept .esp and concept art, when opening up this topic, but I now do not feel that that would provide significant advantage compared to the delay (and work ) that would result in waiting on that content. (I'll still want to make a plugin, but I won't make any promises regarding concept art). Instead, I'll try to make it quick(er):
In my opinion, we need to establish a convention on the use of the OM tileset. In Morrowind, as I've no doubt rambled about several times elsewhere, and will get around to exploring properly later, dungeons had a lot of conventions, touching on things like interior and exterior layout (and no, I'm not talking about cookie-cuttering by Bethesda, though that's also in evidence), and distribution. (Strongholds in a ring, Velothi Domes are not found in southwestern Morrowind, etc.)
Such conventions do not exist for OM ruins, as they were only used once by Bethesda, as part of a unique dungeon complex. TR, though, naturally decided it ought make use of the meshes, and they're now essentially being used for scenic ruins in Indoril lands, with one or two dungeons designed in the set which are generally intended to be epic high-level experiences, essentially.
I actually do not think we need to change this much, just expand on it. My proposal would be to have several types of OM ruins:
1. farming villages;
these would be the precursors of the Velothi/shack hamlets. They'd generally not have an interior; rather they would just be a collection of ruined houses, with perhaps rough scrub where the fields once were, the former crops perhaps now growing wild.
When there are interiors, they would be a series of connected basements never getting larger than a small cave.
There are already examples of this type; in 10,-23 of the Indoril-Thirr file, in [-5, -14] in the Andothren-Thirr north merge, etc.
I mainly suggest this because it is one of the few types of dungeon the OM set is really well-suited for; a lot of its assets are similar to Daedric ruin assets.
These would probably not be shrines to Daedra, as the Dunmer of the time would have probably just used Daedric Ruins. Instead, I was thinking they could be ancestor-shrines, either to a specific ancestor or a whole clan ancestry; something between an Ashlander burial, a Daedric ruin and, naturally, an ancestral tomb.
These would differ from ancestral tombs in being places of worship with large chambers but probably a smaller overall size. Rather than urns, they'd probably feature ashpits, skulls and skeletons. (The sarcophagi).
An OM ruin in Andothren-Thirr North, [-4, -17], could qualify as this sort of dungeon. These would naturally be pre-Tribunal, or at least would have started to fade out with the ascension of the Tribunal.
3. Indoril castle-estates.
There should be very few of these, as most have survived to the current era. I suggested putting one in the Armun Ashlands/Thirr River Valley buffer zone. I personally think there should generally be a story behind why they are ruined, though it is equally possible that some of them date from before the Tribunal, when House Indoril was not yet dominant and Houses often fought each other.
These should have a very grand exterior appearance. Their interiors are more open to variation; in one extreme they could have no interiors, or very small ones, and essentially just function as scenery, while on the other they could have large interiors.
4. ancient House capitals.
I am really not sure about this one, but it is an idea. Basically, some large ruins could represent cities that once vied for power with Old Ebonheart and Almalexia, but were either absorbed or conquered and fell into disuse. These dungeons would often probably be built into the ground, would be very large, and should naturally look impressive from the outside. (And should not exceed the size of large Daedric and Dwemer ruins). Dun Akafell would be an example, though I think they should be handled differently from how Dun Akafell currently is.
Edit: as for the question of who would inhabit the ruins, I'd generally suggest opportunists not directly tied to the ruins, like bandits and vampires. A lot could be quite safe, though, generally being in Indoril lands.
The shrines would be the exception, as these would probably contain ghosts and, at higher levels, Daedra. I also think it would be interesting if some form of ancestor magic could be incorporated in the vein of the waiting door.