Since I liked a lot in FPSC Classic being able to build underground levels I would like to have
the same feature in Reloaded.
From the FPSC Classic manual:
" + Move up a layer
- Move down a layer
Shift+Mouse wheel Move up and down layers"

I guess it all depends how the Construction Kit will finally work. But at the moment without the construction kit building bunkers with the default stock media is a little tricky in the editor when they are multiple floors in height. It's ok if you design everything from the ground up, but once you go up a level it's hard to return to the lower levels again to edit them and place entities etc... Maybe an option in the entities properties where you can designate each entity a layer number. Then you can toggle the various layers on and of.. maybe a bit like in the 3D application Blender? That's quite a nice system... I understand that because it's simple... but most other things in Blender I find hard verging on impossible... hence why I love Reloaded so much... it's nice and simple for the user!

I think a proper 3d editor is essential. Most 3d game engines have them, so it should be possible.