Good question.
Hopefully you get some good responses, and I can relate. Not so much that I want, or have ever wanted to branch off outside of my niche, but I am approached all the time to do projects for other models, and investors. I turn them down because honestly, I do not think I can deliver the same successes in another niche I do not care for, or understand, like I can my main bread an butter one.
I personally did what most here will tell you to do, stick with what you like personally, and sexually. For me, it's foot fetish. It turns my crank, and I have a number of sites in this, and been doing them 8-9 years now. It's all I do, and it's what I am passionate about personally, and professionally, as well as understand. Which is why I am successful.
That being said, I wouldn't have the first clue about ass, or anal, tits, gangbang, blowjobs, handjobs, and so on. I wouldn't even know where to start, what customers like to see in those niches, much less how to market them.
In this business, especially in membership sites, it's a lot more complicated than just throwing a bunch of slop of tits, or ass on a site and hope it makes you money. There are intricacies to any niche out there.
Just like foot fetish is NOT some woman barefoot in a set of images, or video unless feet are the main focus. Or a two second toe kiss in a clip. Or a foot flying in the air during a gang bang. And within foot fetish you have, crush, cast, stockings, nylons, tickling, and on and on. They are not all the same, and even I do not fully understand them all.
I guess the short answer would be, whatever it is you are thinking of getting into, make sure it turns your crank, or at minimum you understand your market or target audience.
