The airplane must have a means of roll control. Without dihedral, you will want ailerons. You could probably design an aircraft that doesn't have ailerons or dihedral, but you will have to have some other means of adjusting the roll (e.g. cg very far below the ac), but you probably won't like the way it handles. Another option would be a v-tail. Then you would have to mix the elevator and rudder to control roll. Its far simpler to just add ailerons.

Brad