A 3D sidescroller would be very cool! I bet Nintendo would be able to create one, if they spent the time and energy to do it, that is. Nintendo is too busy with other games to worry about us fans... :-(
Mario and Luigi for GBA is the closest thing to a "new" Mario game. Super Mario Sunshine was new, but the game just felt like a rehash of Super Mario 64. Same game engine, same moves, same gameplay (besides the FLUDD).