Actually, the formfactor already exists. It's called ExtendedATX. I believe that a specification for it was included on the www.formfactors.org site, but the site is not working right now so I cannot verify it is (still) there.
You are correct, however, it's usage is pretty much restricted to workstation and server designs. Even a full-size ATX tower chassis is not deep enough to fit this board size.
Extended ATX has the same slots numbers and spacing of the normal ATX , I am talking about the slots numbers and spacing between the slots , not the depth of the board.