From what Doffy said, we know that the CD’s have some sort of treasure, furthermore if the public knew about this treasure it would be bad for the WG. Why don’t the CD’s want the public to know about the treasure?
Well i believe that this is because this treasure is actually the ancient weapon Uranus. Now what does it do?? Well i believe Uranus works very similar the the weapon “Etherion” from Fairy Tail
Etherion is a magic weapon that has the ability to create a destructive beam of light that can target any location. I believe that Uranus is very similar; i think Uranus has the ability to launch powerful laser beams which will target any location. This means that even if you are on the complete opposite side of the planet, you will still be inside the range of Uranus.
This makes a lot of sense if Uranus utilized lasers, since Uranus is the God of the sky’s and heavens and light is often associated with heaven and sky. However, i believe their is a downside to using Uranus, that being: Uranus will drain the life of it’s user making them age considerably or die. Now this is the reason why the government needs two devil fruits, these fruits belong too:
First Law; Law has the ability to make anyone immortal. This is why Doflamingo needed the Ope Ope no mi too use immortality surgery on him; by being immortal, Doffy would have the ability to use Uranus without any side affects, making Doffy have equal power too that of a Yonko. The government also needs Bonney; as you know, Bonney’s fruit allows her to alter her age.
This means that even if she becomes old due to the affects of Uranus, she can become a child again and use her power too keep on spamming Uranus. This is why the government wants her; many people speculate the the government wants her so that she can make the Gorosei young again, however i believe that is one part of the reason they want her. It is very likely that their is at least one person in the Gorosei that has no devil fruit and is a haki master;
I believe that the WG wants to kill Bonney and steal her DF so that one of the Gorosei can eat it. If one of the Gorosei eats that DF, they would be immortal since they would never be old; furthermore they can now easily spam the ability of Uranus without having to sacrifice the lives of slaves and other people.