As much as I love the graphical enhancements on this game when modded, I still feel there there isn't much variation in the terrain. Save for the 'Landmarks' mods, is there any mod that makes Cyrodill seem like more than just a big field?

Unique Landscapes, makes all the difference in the world. Makes most of Tamriel seem different (but not all yet).

Absolutely spot on. http://www.gamesas.com/index.php?/topic/1145975-the-unique-landscapes-project/ was born for precisely that reason. Check it out.

Yeah. Unique Landscapes.

I also recommend Elsweyr - The Desert of Anequina mod which adds beautiful and drastically different landscapes just over the hill from Cyrodiil.