I found today some very interesting LotR-themed artwork, quite different from the one in the films but that you can still use for your mod.
First is a Nazgul from Ralph Bakshi's animated films.It is an interesting concept that I think it's worth integrating given that it is from the first television adaption ever of the Lord of the Rings.
Second, I give you a Library with Renders, Screenshots and Artwork (both used and unused) from the old 2003 game War of the Ring, the first LotR strategy game.
Some designs I think have a lot of potential. You might really consider about adapting them in your mod (redesigned somehow, of course, as in the 2003 game they are in cartoonish/warcraft-ish style). For example, the "Orc Mound" seems a perfect Orc Barracks that, given its angular shape, might be re-imagined more in a Cirith-Ungol Style. Upgrading, it gets a real roof and stone wall instead of thatched roof and fence. If I'd compile a list I'd say:
*All Haradrim concept arts for the Haradwaith faction
*Saleme (haradrim hero) might be a good addition to the Haradwaith faction
*The Black Vault can somehow be the Guldur Gaol, but with ruins around it instead of stones and spikes. You described it as an asymmetrical ruin heavily fortified. What I suggest is only to replace the rocks with ruins, leaving both spikes and inner fortifications more or less like that.
*The Beast Lair might offer a good shape / outline for the Spider Lair. Keep the rocks and spikes are those (and maybe even positioned like that). The inner buildings can be transformed in the "heavily overgrown ruins strung together by cobwebs. " you mention in your description.
*The Shadow Lair has design elements (like the combination of dark walls with golden Dome and elongated mouth-skulls at the entry) that fit with the Umbarian Citadel.
*The Slaughterhouse could be a resource building for one of the evil factions, while the Mill can be for one of the good ones. The Smelter and Foundry, are respectively an Evil and a Good building (in War of the Ring), but their design has potential for all factions.
*The Troll Den from the 2003 can be a new Troll Lair for the Misty Mountains or a new neutral troll dwelling.
Elven possible building designs:
Dwarven building designs: