Any mods that majorly change Oblivion's landscape? I can never get into Oblivion because the whole world is just hills and forests. I like how Morrowind had swamps, ashlands, forests, grassy planes, mountains, etc. I know about the Unique Landscape Project, all it does is improve upon the hills and forests.

UL is the only one I am aware of, and 'all it does' is quite a lot, and is a fantastic addition to the game, IMHO.

But no, I don't think there's been a major change in flora and the such by anyone.

Yes, the landscape is rather dull in Oblivion. In Morrowind the swamps felt like real swamps and Solstheim felt like like real snow. And the caves were more "organic", while in Oblivion they are totally barren.
But some of the UL mods do change the look a bit, like Colovian Highlands (or was it Cloudtop?), Dark Forest and Fallenleaf Everglade.

Perhaps you should take a look at the Unique Landscapes thread and browse through the screenshots, eh? "All it does" is add creatively done villages, beautiful new locations, ruins, and adds a very high level of uniqueness to the game's landscape (as the title would suggest, really). There are things like Entius Gorge and Dark Forest that add just wonderful atmospheric locations that you will seek out just to experience them. If you're looking for a set of landscaping mods, you won't find anything remotely better than Unique Landscapes, I can promise you that.

EDIT: Here's the http://www.gamesas.com/index.php?/topic/1076009-relzthe-unique-landscapes-project/ even.

EDIT 2: Some screenshots:

http://i87.photobucket.com/albums/k140/cthulhu314/Cheydinhal%20Falls/UL-CheydinhalFalls-03-Inthewoods.jpg

http://i87.photobucket.com/albums/k140/cthulhu314/Colovian%20Highlands/ScreenShot48.jpg
http://i87.photobucket.com/albums/k140/cthulhu314/Colovian%20Highlands/ScreenShot18.jpg

http://s66.photobucket.com/albums/h271/Phittte/Gorge2Night.jpg

http://i87.photobucket.com/albums/k140/cthulhu314/Dark%20Forest/UL-DarkForest-01-Collage.jpg

http://i87.photobucket.com/albums/k140/cthulhu314/River%20Strid/UL-RiverStrid-05-AHoleintheRoof.jpg

http://i87.photobucket.com/albums/k140/cthulhu314/Brena%20River%20Ravine/02-AyleidTerraces.jpg
http://i87.photobucket.com/albums/k140/cthulhu314/Brena%20River%20Ravine/04-SuicideFalls.jpg

UL is THE best there is...personally, I can't play a game without it. Of course, I am heavily biased...I am a former UL team member/ thread overseer!