I believe that a couple of others have Sniper Elite 4 on their Steam wishlist. Up for sale at the moment. I've debated it on and off for a while as a sort of sandbox rather slow fps (about what I feel I can manage at the moment). What's put me off so far was the cost and the size of the d/l. Reduced cost has made me swallow the download...so to speak. (though I'm now on night 3 of said download, somewhere between 35 and 55gb)
https://www.youtube.com/watch?v=-57lKq4zWvII saw on Rock Paper Shotgun a couple of oddity games.
Fe is a sort of non violent sandbox adventure within a 3D environment. Some reviewers seem to find it too aimless. To me it's odd enough to be attractive (though it only seems to be available on Origin, which is less attractive, it's also a bit more expensive than it should be, I think a bit under £20)
Superflight is a sort of chill out game about falling off a mountain. Ok, it's about using a wingsuit, but you know full well you're going to be a smear pretty soon. It's available on Steam for um... £1.04. Don't let the cost put you off.
https://www.youtube.com/watch?v=nxbOF5LDfEY