Played chess the other day with my nephew (10). We hadn’t played it in a year or two. First we played a regular game.
Then it was his rules.
It got VERY VERY complicated. Started off standard but by the time the game got going, the pieces had individual memory, intelligence, there was a complete trading system, one-use shields, additional special moves…
It was interesting. Completely consistent within its own Universe, just like chess but with many more possibilities available than a chess game
I just waited as new rules came up from him and I absorbed them and incorporated them. Strategy was beyond the 2D world of chess..