Whats your pov about game tech, its age and status? Your example of TPS and FPS being close, is legit, but what happens when the first game is 3D f2p android game and the other is 2D browser furry nsfw game? There are lots of variations to even start comparation, imo..
As the opinion above, itchio gives untied hands for everyone without any harsh rules to follow, combined with its huge content, it's a big pile of data to analyze.
The hidden gems you mention, what would you do with them? How would they serve a purpose in your project? Let's say you found 10 games (hidden gems) which are 99% close to the first game, how would that help your goal? Another example, if all those games are $20 each, i doubt people will happily buy games around.