Well I’ll give my honest opinion here, even if your game is really unpopular - as long as you like it and enjoy developing it you will learn more and be able to create better games in the future (or even the current one may turn out to be a success). If you just aim for popularity, you will get nowhere. And even if you do, it may not last long. So the main thing is that you enjoy what you’re doing, for the fun and experience; not the popularity or “because people will play it”. You could fail to make a game as many times as you like but it will just come to you one day, as long as it’s your passion. Again this is just my opinion, so it is not factual (please don’t criticise if you disagree haha).
This really depends a best way to come up with different games to create or finding one that would interest players is develop a game based on something that you enjoy the most or would love to create.
Find something that inspires you if your into simulators fps games or roleplay create a game based around that role and genre, there’s different factors to go about find something that you would be interested in playing if you instead first person shooter games find a good gameplay create something different that would stand out from existing games or the skills that will employ within the game.
Always sketch or write down different ideas in a notebook, whether it be something that plays a role, such a (Fps, Rpg, Simulator, Adventure, Action ect). It basically comes down to your overall preferences.
Find something that you would really have a fun time enjoying - playing and that users will have a better player experience looking at existing games is another to find games that player spend the most time on there’s different approaches to take.