The way members have made this happen in the past, if I am not mistaken, is to accumulate a list of threads to populate the potential new forum. Once that number exceeds a certain critical mass, then a new forum is born.
Forums aren't just added will-ye, nil-ye, because such decisions are heavily influenced by their potential effect on the amount of GoogleLove this site gets. As soon as we have enough of the criteria for a new forum lined up, such that the changes don't negatively affect how we are perceived by Google, or in fact positively affect us, it will happen.
Good luck, though. Get the go-ahead from one of the staff before starting work, because they might have a reason for not already having such a forum. Let us know how you do.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein