So your plan is to rely on the players being the curators. Might work. But it will be super hard to re-invent this. This is going on and being developed and worked on for at least 30 years or so. https://en.wikipedia.org/wiki/Firefly_%28website%29 https://en.wikipedia.org/wiki/Collaborative_filtering
Even harder will it be to get users to use your platform. I see threads like your's here every other month or so. Someone comes along and wishes for a new place for people to find games, or to promote games. Might work out (it did for Itch, though the motivation to create Itch was a bit different.). But most do not work out. I remember a longer discussion with someone here that had nice big screens of mappings with how corelated tags were or something like that. I do not remember the specifics, but I think it was the same overal concept of somehow deducing what a game's target audience is.
The hardest selling point might be, why use your platform, if they can just use one of the already existing ones, like igdb. Including the actual platforms, the games are available on. Most people are perfectly capable of finding new games to play on Steam, by the features of Steam. Itch has some a lot of quirks, but even here you can learn how to find games you might like.
To pitch as a service that you can match a game to a player by mood or vibe, you have to have a metric of what the game is and more importantly, what the player really wants.
If you accept developers to submit their own games, you will need to decide if your goal is to have a discovery feature for professional games or for hobby developer games. There is literally no entry bar on Itch. So claiming to be a published developer here, would mean nothing in terms of professional level. I do not know how igdb handles this. But I doubt you can submit a fresh game published yesterday and only on Itch by a hobby dev.
When you say hidden gem, I would think of games like this https://www.igdb.com/games/vambrace-cold-soul . Interestingly the one critic did give it a 9. While the people that bothered to rate it only gave it a 6. It's basically a story rpg with the turn based combat mechancis of Darkest Dungeon. Still some people call it a roguelite or roguelike, even on igdb. It is the 7th most popular tag for this game on Steam. But this is rogueish the same way any rpg can be a bit different each playthrough. But it lacks the "run" mechanic any rogueish game has - it is not meant to be played several times. So using that rogue classification to recommend the game to players, might fail utterly.
And the issue is, that even somewhat known hidden gems, just have very few ratings. You can't do much statistic with only a couple reviews.
Speaking of Darkest Dungeon. They made a sequel. The sequel has about 1:3 negative ratings. The first one has about 1:10. What you do not see is all the people that were wise enough to skip the sequel. The sequel also only has about 15% review numbers compared to the original. Superficially, they share the same tags. But everything that made DD1 tick and resonate with the players, they took away from the sequel. The issue with roguelike and roguelite is, that most people cannot distinguish between those and often use the terms "wrong". But dd1 was roguelike and dd2 is roguelite. One is full of careful planning and investment and dread to lose all that, the other ist just grinding.
So what your thing ideally should be able to is this: see what makes people like dd1 and recognise that they probably will not like dd2.
And as a counterexample for sequels. Monster Train 2. They took everything from MT1 and improved it. MM2 has only 50% of the reviews, not 15%. But most importantly, the ratio of negative ratings on Steam is about the same for both games. It's 1:20. So I dare say it gives the same vibes to the target audience.