I'd like to have a "Curtain of Darkness" element. Basically a wall-like board with a blackness pattern on both sides. It should be 2x3, like a small wall, maybe just 1 paper-thickness smaller. No need for clip indents on the sides, but at the bottom there should be 2 indents that are deeper than usual in a way that when sitting on 2 T-clips it is level with the floor and the Ts leave enough space in the middle for the I-clip of the floor tiles below. (I will need to add a drawing, I guess...)

Those "Curtains of Darkness" could be used to clearly mark where the Fog of War starts. Or just to provide a nice backdrop for an open door when making photos.

"Tiles of Solid" would be just normal floor tiles but with nothing (black) on it. Or maybe even a big hole in them. They'd be to mark areas that are not accessible, e.g. solid wall. Most times you can just not use a tile there, but sometimes the build need the stability.

Go to a crafting or framing store, buy a piece of black mat board, and you can make these yourself with a craft knife and a little time.