I recently discovered the Banana Ball exhibition baseball games, and their custom ruleset, featuring limitations, crowd participation mechanics and special roles among other things.

This reminded me of (and it's an derivative game rather than an alternate ruleset) Three-Sided Football, which, among other things, is a Situationist, philosophical and sociological rabbit-hole.

I also recall dark chess, a chess variant with line-of-sight mechanics, to emulate the fog of war. There are thousands of chess variants stretching back a thousand years, this is just one of the first I learned of which really interested me.