I know there are getting to be a lot of sub-forums, but gravel grinding and similar off pavement endurance events are becoming extremely popular. Several companies are putting out bikes aimed directly at that market and new events are popping up all over the country. In MN alone there are the Almanzo, the Royal, the Alexander, the Woodchipper, and Heck of the North that I am aware of, probably more. Off road trail and gravel backroads group rides are also becoming popular.
As the sport is significantly different that either MTB or CX, perhaps it would support a sub-forum of its own.