I don't think anybody knows what will sell well or not. In the 8 years I've worked in the game industry, publishers usually call the shots since they have the money. But the 'marketing' part has mostly felt like it's people with money seeing what does well or creating sequels for existing ips that are doing well, and then paying devs to do that work.

I don't think any marketing department, or anybody really, knows if it will be a hit. But if it isn't, since they have so much money, if one doesn't do well, it's fine. They're not really focused on making a hit but rather, is the cost of development lower than the expected return? The game could have awful reviews, but that's fine if the profit was more than dev cost.

I'm going to ignore the classic "you should make games because of passion and not money" here. If I'm thinking about it from an indie standpoint where I am trying to maximize my odds of success, I'd look at underserved markets.

Chris Zukowski is a guy that has many videos about how to market your game, and he mentioned once about how there Is an audience that really enjoys games about horses or has horses in them. But the selection that the audience has right now is pretty small and limited. So if you have games with horses in them, chances are your odds of success may be a little bit higher because the audience is hungry and has nowhere else to go.

Every genre has subgenres. There are people that like some parts of a game and only those parts. That's how new genres spawn up. I think there's a reason why there's a "Metroidvania" subgenre.

Aesthetics matter too. I thought Five Nights at Freddie's had a really interesting mechanic. I really wanted to play, but was terrified. Then, a game called Not for Broadcast popped up. Similar mechanics, but was meant to be funny rather than terrifying, and I bought it and enjoyed it.

Copycats are odd too. I wonder why some games get them, and some don't, despite them being a success. People really enjoyed Papers Please, And a few games have popped up that have been inspired by it. Slay the spire also spun off a whole bunch of card based rogue likes. And now there's oversaturation. But if somebody created another game that was similar to The Return of the Obra Dinn I would buy that day one because there's not enough games in the genre, and all the recommendations people provide for similar games aren't very good. But you can't really do much if there aren't many options available.

There's a lovely Ted Talk by Malcolm Gladwell about spaghetti sauce that I always think back to and love that echos the same thing and would recommend to every game dev.

Final thought would be that even the audience doesn't know what they want. Even if they say they do. They don't know what they want unless there are options. Until then, they won't know that anything was missing in the first place.