A drivable warthog would be great for really bumpy off-road maps, no doubt, and like you say, only having an autovista of a vehicle we already know about isn't much of a give-away for Halo 4. Maybe even as DLC the warthog could be included. But then , you have to think about what Forza is, a serious racing game, and a gimmick like a warthog wouldn't be very serious.