I think a forest would look awful. Halo looks like complete crap with its basic ground textures. Without an engine like Halo 3 and it's RBG blending, you can't make a map that's grass and rock look good, especially when it's huge.

A desert would only look good if you put enough effort into making a ton of stick trees. You'd have to emulate The Ark as best as possible, and nothing short of that.

I actually still like the idea of snow. The waterfall would still be able to stay, but in the distance, you would see all ice glaciers and stuff. Think about how Avalanche is in Halo 3. The map is beautiful. Snow would be good if you placed snowy trees throughout the map, although I don't know if Halo, being so young, would have fully grown trees within a few weeks of time. If you look at Avalanche in Halo 3 and aim for that beauty, you can really make it look nice. Likewise, if you target The Ark, you can make it look beautiful that way too.