Seriously... when he said he had an idea for something simple, literally my first thought was: "Is it Othello again? Is it motherfucking Othello again?"
And of course, it is motherfuckin Othello again.
Do they even fuckin play Othello in Japan? Why is it so damned popular in isekai series?
I used to have an Othello board I'd take to the pub with me to play against random people. Was great fun. Only two ever managed to beat me though, and one of them I had about equal wins and losses with. Problem is pieces kept going missing... as happens when you play board games in a pub... so eventually I didn't have enough pieces left to play a full game with.
ANYWAY... the point I originally intended to get at is that it was a pretty weird and niche thing for me to fixate on, since it would be more usual to play chess, checkers, backgammon or a card game of some sort... Yet in these isekai series, othello is somehow more common than chess, shogi or go, let alone checkers, backgammon or any sort of card game... and there is never any reason given for it.
Also they're calling it "Reversi" ... the inferior name.... but that is just a matter of personal tastes...