My theory is that Rocks D. Xebec was the previous user of the Goro Goro No Mi (Enel’s Devil Fruit). In general, the Goro Goro No Mi is considered one one the most devastating devil fruits and I feel like Rocks would have been able to use it to its fullest extents. For example, he could have been the one who created Raijin Island.
When Rocks died in God Valley, the island disappeared and with the devil fruit having nowhere else to go, went up to Enel’s hometown. With Goro Goro No Mi out of the Marine’s grasp, they began to look into how they can counter Goro Goro No Mi should it fall into the hands of a pirate again.
So we come to Gomu Gomu No Mi. In the Chapter 1017 we learned that the World Government/Marines were transporting the devil fruit to an unknown location before Shanks stole it.
I think the World Government was trying to secure the Gomu Gomu no Mi so that they can have a hard counter to the Goro Goro No Mi should it reappear again.
The reason why I like this theory so much is because I don’t like the idea of Luffy’s devil fruit having some special power no other devil fruits have. If it indirectly held some special importance, that’s much better in my eyes.
Setting some foundations first, I don’t believe in some of the popular theories such as the Gomu Gomu No Mi fruit being Roger’s previous fruit or Rocks’ fruit because it would’ve been mentioned by other characters. Also rereading Chapter 1, my impression was that Shanks only thought the Gomu Gomu No Mi was important because it was a devil fruit – just regular devil fruits are worth a lot – not because it was special in any way.