Everyone knows them: mobile game genres.
Genres like your Candy Crush-like Bejeweled clones, Flappy Birds, Clash of Clans, Tiny Towers and many more practically only exist in the mobile game space. There are even game genres that don’t exist in practice at all, and only serve for fake gameplay ads.
Interestingly enough, virtually all of these are exclusive to predatory, microtransaction- and tracker-ridden mobile schemes for casual vulnerable players: non-tech-savvy and/or young people.
There is almost no competition in those genres for non-predatory games.
The question is: are these genres inherently predatory and money focused, or can they be saved and libre-fied? It would maybe be an effective way of bringing people who are being exploited right now to the libre side by supplying an obviously better product. Only requirement is presenting them in a similar way, not with the typical FOSS clunkiness like Minetest.
I think the only issue is this will need to be in-app payments, I don’t think f-droid supports paying for an app outright at the moment. Some developers might not want to make their own payment or software key infrastructure, something the app store solves
Or they could just redirect to already widely used services like Patreon, ko-fi, Monero, or a PayPal address. Doesn’t have to be more complicated than other apps. :)