a image gallery that can also play video media would be nice (for high quality super awesome porn you find on the net). theres some porn out there that just makes your jaw drop and collecting that and hosting it on your website amazes people. I've had websites in the past using a simple machines free media addon that did this very well. its really the only thing i have lacking on xenforo.
I'm sure there are plenty of places where people can get that porn so there's no need to worry about providing the service.
Commercially it's a dubious idea as there is far too much competition. A lot of it is run by some definitely nasty people based in Russia who you would not want to annoy.