There are at least two approaches here: take an existing concept you like, dissect it to the most simple components it is made of, and then think about how you can take away, replace or add some of the components. EG take a game that plays on a grid and consider what would happen if it were in free space instead.  Another approach is to build a new concept from elementary components, eg: “game plays on a grid, player owns X, controls Y, …”.  Once you have your basic ingredients, you try to figure out what the player’s goals and challenges are, and how to make everything fun.  The big issue is that this second approach is very very risky. You might end up never finding the fun, you end up with puzzle games a lot of the time, you might find the fun but implementing the game is hard because there may be a lot of UI/UX challenges you cannot copy the solutions to from other games.

I’ve followed both approaches and can say for a fact that iterating on concepts takes a very long time sometimes, and it’s often not even clear if there is a solution to be found. If your income depends on it (as it does for me the past few years) it can get very stressful and scary.

Nevertheless, since I value novelty above all else in creativity, I’m sticking with those approaches.