There are so many variations of the various types of games it is difficult to narrow down. You should probably pick a general style you would like to develop then deal with the different ways that style can be implemented.
Open world vs platforming (and open world can be implemented with a platforming style). Single player vs multiplayer vs MMORPG (they can all have RPG features so you have to decide if you are interested in RPG at all). Then you can decide if you are interested in sports, military, action/adventure, fantasy, etc.
If you are interested in developing the game engine yourself, there are the more specific features to consider, like how the geometry is created and how much detail it has, what type of object animation you will support, physics, scripting, etc. You also have to consider if you want the game engine to be specialized for a specific type of game or if you want it to be more general purpose (and therefore more difficult to program).
There are many things to be considered and you didn't indicate if you are interested in general game design, programming, modeling, etc. If you can be more specific it will help us to help you.
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