Best sandbox non-games that would be great for a real game

[rusty_shackleford]

These sandboxes tend to have a lot of cool mechanical depth and complexity, but they forgot to add a game. D'oh!
Which sandbox do you think would be great for an actual game to be made in? What kind of game?

I'd like to see something like Underrail or Fallout made in Project Zomboid.

[rusty_shackleford]

Maybe the unmodded native, but practically anything else has Kingdoms destroying one-another

The mountain blade sandboxes would make for good games

[sheet]

They made a sandbox in that sandbox: