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.
As a kid , in the midwest winters, late 90s, we would play a lot of games inside. With cousins or siblings.
Usually too many people to play traditional games with.
So we would double the game components and invent new ways to make it work.
One I remember the most was Rummy 10,000. Or that’s what we called it. Not sure if it was infact rummy 5000 but altered. I think it did have some of the traditional rummy 5000 rules but it wasn’t quite the same.
We played with two decks of cards. One black. One red. There was some rules or something about the deck color.
I honestly can’t recall the extra rules off the top of my head. But I do recall it was very fun. 6-8 players.
https://en.wikipedia.org/wiki/Continental_Rummy (Double deck rummy)
https://gamerules.com/rules/5000-rummy/ (Single or double deck)