“hidden gems”
That is highly subjective. One user's gem is another user's trash.
If I understand correctly you want an algorithm to detect underrated games. Not in the factual sense that they have a low number of ratings. But in the sense that they should be more famous.
I do not think you can do that with tags. And not really on Itch. One of the best games I know on Itch in a special sub sub category. It does not even have a tag. (I shall not name it here, but let's say, I knew that game before I knew Itch, so to me, the game is more famous than Itch itself.)
To give a real world example. https://tomorrowcorporation.itch.io/human-resource-machine
The game has 4 ratings currently. It is from 2018. Has a single comment ... from 2025.
The plot twist is, the game has 3000 reviews on Steam...