Not a gamedev myself, but I think playing a bunch of board games is huge— the complexity constraints for a board game are way tougher than they are for a computer, so designers (the good ones, anyway) are really forced to consider if each new resource, token, turn phase, victory condition, etc is really pulling its weight in the final product. The mechanics have to be
ruthlessly stripped back to be just what is absolutely essential to make it work. Games like Patchwork, Santorini, Azul, Carcassonne, etc are a master class in this.
A computer can be doing mountains of bookkeeping behind the scenes, and that not only obscures what is actually going on from the player, but it can make the designer believe they have "depth" when all they really have is an overcomplicated mess.
Watch YouTube channels where board games are taught, and pay attention to the structure of a video like that— it's "you play as X in setting Y, your ultimate goal is to Z, and along the way you're going have to A, B, and C to achieve that. The game will end when conditions Q are met. The hook that makes this game unique is that ____."
A video game (especially an indie game) should be able to pitched exactly as succinctly as that.