The most interesting theory I've ever heard (and you probably have too because it's quite common) is that Gol. D. Roger had the Gomu Gomu No mi and used Gear 2 (or the equivalent tactic when he had it) too much which lead to his disease and later to his death and that Shanks took the df as tribute and therefore that's the reason Shanks had the Gomu Gomu No mi and the reason that Luffy ate it
What do you think?
I think it's kinda far fetched and unrealistic but it's definitely fun and interesting