My suggestion is to allow factories to set their waypoint to a unit group. When the factory finishes constructing a unit, it'll receive the same orders as the rest of the units in the group. If there's more than one order, it'll assign one randomly (or maybe by unit type)
The reason I want this is because then I can setup air patrols to be continuously replenished by a factory. Right now if you make a patrol order from a factory then give an order to the patroling units, there's no way for them to rejoin the patrol again. But if a unit group had the order and the factory was just assisting the unit group, then you could give the units any order you wanted and the factory would create more units for them.