The jist of this theory is that Shanks is collecting Devil Fruits and storing them somewhere (Red Hair Pirates’ Headquarters?) to prevent people from eating them and using them for “evil” purposes, whatever evil purposes may be in his eyes.
The following points are things I have noticed in the story that support this theory.
1. Shanks does not have a Devil Fruit
Shanks is the only Yonko without a Devil Fruit. Does that not sound strange to you? Maybe he’s so strong he doesn’t need it? I think so too, but what is strange is that we have no devil fruit users confirmed on his crew, a Yonko crew. I find it really unusual that Oda has not shown us anything that even hints at it, because we know he is very good at foreshadowing. The closest thing that we have seen is Lucky Roo’s “teleport” in Chapter 1 but that can be easily explained with Haki or Rokushiki.
2. Buggy… just Buggy
Buggy is the only confirmed Devil Fruit user on Roger’s crew. We know that he did not eat his devil fruit on purpose. He still blames Shanks for it to this day which might mean he is not happy with his devil fruit powers and maybe would rather not have them. So far we haven’t seen another devil fruit user on Roger’s crew, surely they knew something we don’t know yet.
3. Blackbeard vs Shanks
Then we have Blackbeard, who is basically doing the same as I suggest Shanks is doing but in a different way. His way is more stealing the Devil Fruit powers and using them for his crew and himself (Whitebeard, Absalom,…). Shanks should be aware of this fact and has always had a rivalry with Blackbeard that might have started years ago (Shanks’ scar). Also in his talk with Whitebeard he pointed out that Blackbeard is really dangerous, maybe because he was aware of what the Yami Yami no Mi or Devil Fruits in general are capable of.
4. Gomu Gomu No Mi
Ok, let’s talk about the Gomu Gomu No Mi. Why did Shanks steal it? Maybe he wanted to add it to his collection of Devil Fruits so no one could use the power. When we see it in Chapter 1 it did seem to be locked up very tight, which makes sense if the World Government finds it so important. He also seemed mad at Luffy for eating it, maybe he knows that devil fruit can be really dangerous and didn’t want Luffy to have devil fruit powers. Or maybe he wanted to sell it to the highest bidder? Who knows? Maybe Who’s Who does?