A Role-Playing Game without a Main Quest? Sounds a bit like a MMORPG. The only difference being that MMORPG's are mainly driven by the concept of the "Holy Grail" (Background Story) For a Single-Player Role-Playing Game, scrapping the Main Quest is a bad idea and will not impress, because the Main Quest is the other drive that the player needs in order to keep the player interested, and to influence the choices you make for your Player Character. No Main Quest means no interest, and no true development of the Character you play. Wandering aimlessly without a main goal is pointless. Even if the "Holy Grail" is there in Fallout, it cannot stand by itself so simply without a guide for the Player Character.