Love this the most!
Thanks for the suggestions guys! 
I’ve surrounded my level with a minefield, a river, a dark wood (with a monster) and some animals. Anything I’ve missed?
You’re on an island(water surrounding you) and you can swim, but it’s an endless ocean.
A mist-dragon farting unicorn?
Actually - mountains. Just for hilarity’s sake.
Creative level design subtracts from the cheap feeling of an invisible wall.
Unscalable slopes, debris, wreckage, etc. We use things like that couple with our Boundary tool to manage the box collider’s instead of mesh collider’s or coding for slope limitation.