Morrowind was amazing because Vvardenfell was such a unique and alien setting that fantasy gamers hadn't seen before. Barren harsh landscape, dust everywhere, giant mushrooms, multiple moons, drab color palette somehow strangely beautiful, locals hate you. Huge success, and to please all the casual fans that were gained Oblivion was pretty much just a generic medieval times setting in Cyrodiil. Loved the game, that's not my point. Skyrim is a more interesting setting, but it's still just kind of fantasy Braveheart (Game of Thrones?) style we've seen before.
You have these two incredibly unique beast races in your games/lore (we've seen Orcs in a million games) and I think it would be a good idea to make them the focal point of the next game. Black Marsh borders Morrowind and Cyrodiil, perfect for tying in storylines. Please give us a dark, dirty, gritty, swampy Argonian game. I realize Black Marsh is pretty isolated and there wouldn't be a lot of diversity there as far as other races, but that would be cool. Back to Morrowind style where the local population hates you and doesn't want you there.
I would also settle for bounding through Elsweyr with my clan of Khajiit on the backs of giant feral cat mounts. You've pleased regular high fantasy fans the past two games, now use your unique beast races to please the hardcoe fans while surprising casual fans by giving them something they didn't know they wanted.