Does anyone know of any mods that modify Mamaea (aka, make it a more interesting dungeon crawl like Darknut and Trainwiz did for the other ash vampire citadels)?

I don't think there are any. Not even MW Modding History yields any results. I guess there's an opportunity there, then, for anyone with the time/interest.

Certainly http://mw.modhistory.com/download-24-10326 added some extra NPC's with custom gear to Mamaea but didn't really alter the lay out.

I'm thinking of trying my hand at this. Anything in particular you want to see? Place is home to the only Ash Vampire outside the Ghostfence, Dagoth Araynys, and it has the curious bone texture in some parts, so potential is definitely there. Maybe there's a Dwemer ruin down there, and Ur has tasked him with digging it up and securing it?