Game design ideas gain value, sophistication, and meaning only when they are playtested. The hard work of the process, the actual game design itself, happens during the iterative process, not the concepting process that proceeds it.
Finding good playtesters is hard, and playtesters are probably underrated as to how much they contribute to a game. When I started out designing, I relied on friends and anyone who didn’t run fast enough.
Historically games were less designed than evolved. Not many of these games are credited with a designer, and for those games that are, the contribution of the “designer” is usually a small variation of an existing game.