What is the key for, Doflamingo is holding in chapter 752? As far as I recall, the only locks mentioned during the Dressrosa Arc are:


The lock to the SMILE factory
The lock of Law's handcuffs


Though, in chapter 747 Doflamingo destroys the key to the factory.



And in chapter 750, we can see Viola holding the key to Law's handcuffs.



So what is the key, Doflamingo is holding in chapter 752, for?
I think as far as we know, you can't tell for sure if it's one of these two possibilities.

But if I compare the key to the SMILE Factory with the last one you can't see anything similar. So I think it's not another key to the Factory.




Even though the other is much smaller on the picture, if you zoom in, you can see that the key for Law's chain is round. On the other side Doflamingo's key has the shape of a heart.

Another point is, that the left one has a little hook...




So I think this is just speculation, but I think even if it's one of these two options it would be the key to Law's chain.

In my opinion, it's a key to something new, maybe even more important.