Ahh, a primary favorite. "Gravity is the grand leviathan of forces". It is my personal belief that, as light and energy were shown equivalent, the third leg of the stool will emerge; that gravity and mass are equivalent. Not as we understand them now; but as mc^2 understood light/energy equivalence.

Final thought exercise: is it possible, that gravity does not exhibit the same behavior everywhere in the universe? That gravity may have states.