Game design is kind of like an onion.
Take my game, Warriors from the Mystic Mountain. There’s a whole lot of stuff going on. I can’t really playtest it effectively at this stage, since the core rules and the special abilities are simultaneously in flux. What I really need to do is nail one down, get a stable core, and build outward from there.
Chi powers, and chi itself? Key to the game, but built upon the combat rules. Same with passion. What I really need to do is dig in and make the Ebb and Flow positioning really grabby and tactical, and go from there, and then build out- adding layers back in, testing each one of those, to make sure it works.
The resulting playtest probably won’t be fun, and it won’t look like roleplaying at all, but more like making a board game. But it really needs to be done.