I think this is nifty, and a really great solution to a bunch of tutorial mode problems.

My main worry is that it will take away from the lonely castaway feeling of the game. And it's only a worry, a point of concern. I'm not saying that you shouldn't do it, just that you should consider this ...

Can I suggest to call shallow water bogland or marsh? Then it makes more sense you can walk on it but not build on it.

An intuitive idea is to have two types of landfill:


Basic Landfill: makes water into bogland
Permeable Landfill: fills in all water and bogland

Basic Landfill would be ...

Sounds good, but for UNDO I would restrict it to current screen/view only. Even with long reach settings.

I imagine situations, where I press CTLR+Z and nothing happened. Nothing near the player...
But far away, in remote factory outpost, some crucial infrastructure element just gone out of ...