MFF is just an example I used anyway. You can pick whatever other fetish you want that has fairly broad appeal. As an example I have a mod for Babysitter that started out purely as an expansion centered around the male MC but got so many requests for NTR that I decided to add an NTR path as well even though I personally hate playing that path and in fact started the mod specifically because I found the base game's NTR somewhat annoying and forced. Ironically I actually found that while I hate playing it I actually don't mind developing for it. If I hated both I would have refused to do it. That's what I'm getting at. Give your customers what they want but only if you enjoy developing it as well yourself.The main market for lesbian porn are straight men, simply because there are more straight men than lesbian women. Even if only half or merely a third of us, prefer lesbian porn, we are still more numerous than lesbian women. MFF threesomes, though I think, are about equally annoying to lesbian women and straight men on average, though I personally certainly hate those anticlimactic scenes more than any lesbian woman here does.
A major problem might be that many developers still think, a game has to have a "relatable" protagonist for the main male audience.
If everyone wants foot fetish stuff I'll take a stab at it. Doesn't do anything for me personally but it doesn't repulse me. By contrast if everyone was demanding MM, Futa, tranny, or some other fetish that I find repulsive, forget it. Not happening no matter how many people want it. So it doesn't necessarily have to even be anything involving any guys. Maybe there's unmet demand for bondage or toys or cosplay or something while still being pure lesbo. Pick something that your customers want that you're not opposed to. Or just accept a small, niche market...