Morrowind was marshy, Oblivion was lush and green and Skyrim was mountainous and wintery. It stands to reason that Bethesda may want to continue that trend and set the next game in a unique biome, rather than return to one that's too similar to what they've already done.
Desert would certainly make it distinct from previous iterations. There's the danger of it being bland, as you said, but there are possibilities besides just dunes and flat ground. They could have canyons (like that one DLC for Fallout: New Vegas), large oases (where things are lush and green, for a nice break), snowy parts (since a lot of deserts are awfully near higher elevation areas that get snow) and even just lands that used to be lush but are experiencing drought. On the other hand, I just realized, a desert biome might make it seem too much like Fallout's world, so maybe they would be afraid of that.
Jungles would be distinct, as well.
There's also the option of just having a world sort of like World of Warcraft's, in which all kinds of biomes are available. You could have desert in one corner of the map, jungle or marsh in another, mountains in another and green hills in another. That's personally what I'd prefer, since I prefer variety, even if it's not terribly realistic. I'm not sure how they'd make that work in their world map, though. It's always puzzled me how they limit their creative options by defining the geography and climate of lands long before they set games in them.
... which of course lead us to the Oblivion level scaling system 
