If you want to keep your idea to yourself it's fine, but anyone who wants to share is welcome.

Here's my idea:

Taking what's addictive and fun about a game with predatory monetization and stripping out any predatory element. Like a gacha game but singleplayer and with no microtransactions at all.

What I mean by that is that predatory games are very fun. There's something about them, the way they reward you and hook you, that makes people want to spend so much money.

Well imagine a game where you CAN'T spend anything, but it's still designed to be as addictive and fun. A simple one-time, offline singleplayer purchase but with the fun of one of those games.

Why would anyone make this? Well, a big company won't, it would want to squeeze every dime. But an indie dev might want to make it for the one time fee of $15 or whatever. Many indie devs would be happy with that.