Psy, most of the features you listed are things that players can build themselves in sandbox, or settings you can change with tools (anti-gravity, ice, etc)
As Eclipsed mentioned, adding too much will make it an LDT-only server, and discourage new people. Even people from LDT would probably lose interest after a while as well.
It's kind of like.. what would be the reason to play a pokémon game if you started out with the pokédex filled and had your PC full of level 100 pokémon? Basically, everything that the mapper creates are things that the players won't bother with, so everything that is created will further decrease interest in the game mode. Make a town in hammer editor, and people who like to build structures won't join. Make a race-track in hammer, and that will be the only track people race on, etc.. People will get bored.
(also, players rarely want/need terrain to play on, but when someone gets the urge, the 3d skybox provides a nice hill landscape.)
Some of the things you listed are hard to make in sandbox though:
Underground area - I've already built a tunnel leading underground from the main area, but there's not much down there atm :3.
I suggest to make it into a smaller building area where people don't have to deal with the above-world chaos
Caves & Underwater caves - this isn't really about building, but about exploring an unknown world. if you build it yourself you'll know it all by heart, so I can understand why this would be made in hammer. I actually made some research in how to make underwater caves in hammer, and it could be pretty awesome, but a looooot of work to make it really feel underwater, since underwater life is pretty complex.
What would be really awesome though, is that all areas could be connected.
flatgrass area <-> underground tunnel <-> underground area <-> cave <-> cave river <-> waterfall <-> underwater cave <-> underwater cave exit <-> flatwater area <-> beach <-> flatgrass area
Yep, there's plenty of room. 50-55% skybox height is enough for planes, that's about 350 meters. then the 3d skybox takes up some vertical space... but yeah, room is not an issue. But we should make sure that what's added actually serves a purpose in sandbox and doesn't discourage general sandbox gameplay.