Firstly I think there should be a possibility to make underground bases/bunkers that would add more possibilities to make an base than just having it on the ground and to keep it balanced you would need to find a way to get air inside of the base/bunker
I think there should be more depth to buildables such as elevators,moving platforms,openable roofs etc this would open up a huge load of possibilities for making bases
I mean, making bases is caves would be pretty nice, especially being able to build in a cave without excessive effort or F6 edit mode, but I think digging into the ground would be a bit hard to implement.
A solution could probably be that you can place objects under (as in say you put a floor under the terrain), and the terrian inside it disappears, like the landscape holes in the devkit for 3.0. When it is destroyed/removed, everything inside of it (including players) dies.
There definitely needs to be limitations on how far you can dig.
There should be a substantial protected area around spawn locations.
Just as buildings decaying in 3.X helps prevent that, holes refilling would help prevent that in 4.X. (I think some people have some very strong opinions on how exactly building decay needs to be tweaked for 4.X, or what it cannot be changed to, so I’ll just leave it at that.)
I think a good idea in my opinion is not being able to dig anywhere at any depth, but have certain hills or mounds that you can dig up but there is a set amount of depth that you can dig, plus this could open up certain easter egg possibilities. And elevators and such I have been thinking about a lot and i think it’s a good idea.