Because I do game dev as a hobby, that means it's fueled by me wanting something to exist so badly that I put in the time and effort to make it real. So with that in mind I mostly want to make stuff that either doesn't quite exist (either in the form I'd like, or at all) or I want there to be more of something I'm passionate about.
A tricky aspect of that is the fact that I inherently can't have the experience of exploring whatever I make from the perspective of someone who's never played it before, and I can't discover the secrets and want to see where the story is going to go next because I know all of that already. Though sometimes looking back at a game years later when I've kinda forgotten some of it can make it kind of fresh.
But I don't think that's necessarily bad? When I really enjoy a game I'll end up at a point where I know all about it, and know all of the secrets but I still have fun running through it again and again because it's that fun, all while seeing myself improve at the game. And when you make a game, you kinda skip forward to that point. I won't have that feeling of discovery but I'm not sure that's wholly necessary to enjoy a game.
Though I guess that definitely relies on whether or not a game would be fun to replay, and not everything is. Predefined puzzles you know the solution for (because you made the puzzles) aren't really going to be that fun to solve, and something that's built entirely around seeing the story might not be that fun if you already know how it goes (though maybe it can be like rewatching a favorite movie or rereading a favorite book?). But I'm mostly into making stuff like platformers and action puzzle games, and those are great to play over and over again, so that's fortunate for me.
Aside from knowing about secrets and stories, I can also think of games where part of the fun is just kind of, figuring out the rules? Or where the rules aren't really meant to be known, and it's supposed to be vague so that you can do what makes logical sense and it will work out. RollerCoaster Tycoon is a good example of that; it's a very complicated simulation and Chris Sawyer included a really impressive amount of details and mechanics, and you really can think of it like a real theme park that works in a way that makes sense. Once you actually know about the underlying mechanics it kind of turns into a different game. Efficient RollerCoaster Tycoon play seems to focus on raising a number that's never displayed to the player nor mentioned in the manual at all.
My mind has been on this lately because I've been thinking a lot about wanting to make a virtual pet. I have a lot of thoughts on how it could work, and I'm not sure if the kind of stuff I want to do really exists in the way I'd like. But for a virtual pet you kind of want to be able to pretend that it's an actual critter, not just some meters that go down at predefined rates, and it feels like it may be tricky to have that when you made it.
I dunno, maybe if I actually tried it I'd find that it's actually fun to pretend even though I know how it works. I know how Tamagotchi works and it doesn't make it less fun, and knowing that a meter is going down at a specific rate doesn't make me not enjoy checking in on a critter and taking care of needs I see pop up. I'm guessing part of this is going to be the human tendency to anthropomorphize things. It feels kinda cozy even just like, inviting my Discord bot to a call to watch a movie with me and friends even if I fully know nothing like that is actually taking place. So I'm not too worried.
Ignoring the rules of the simulation itself, there's still room for random chance (seeing what shop items are available, seeing what characters you'll get, random events), and just the feeling that you did a good job, or finally got the thing you wanted.
Of course you can still have that feeling of discovery and not knowing how everything works from playing other developers' games. I really appreciate that there are lots of other people who are interested enough in animal mascot platformers games in order to make more that I can try myself! They won't necessarily make the perfect game for me, but there's still going to be a whole lot of fun to be had, and I get to have that cool feeling of discovery with a genre I'm passionate about. And then they can play my own game and get to discover and explore with my own creation.