I asked ChatGPT for a game idea the other day. It replied with several ideas and I iterated through “I like the genre, can we change the theme”. It eventually landed on something I liked “awesome. Can you give me a color scheme in hex values”. Returned the list. “Awesome. What materials should I use?” Returned a list. “Can you explain how I might start on building the mechanics”. It gave a decent idea, but all in all I knew where I was going to start anyway. “What could I sell in the game?” Returned with some ideas. “What multiplayer features could I add”. Returned with ideas.

TLDR: ChatGPT did an entire weeks worth of brainstorming and game design preplanning in about 10 minutes.