I develop games in my free time for fun. But I find it quite demotivating when I find that the idea I thought to be original already implemented years and decades ago. Even if I cannot find an example by searching the web.
So it's not worth making a yet another one, because it would be the same old boring thing.
So is it on-topic to ask whether a game exist that implements a particular idea or combinations of ideas?
Is there an FPS game where you can build a base to defend your team? Yes there is: Tremulous for example.
Is there a game where you have 6 degrees and freedom and play in a closed level? Yes there is: Descent.
Of course these are well known, I would ask about different combinations I couldn't find examples for.