In chapter 906 we see a lone figure, carrying Luffy’s new wanted poster, opens the door to a freezing vault. Stepping inside, the vault is revealed to contain a single object: a massive straw hat the size of the figure.
I believe that what we saw in the room was actually Roger’s grave (the “table” in the middle of it could easily be a coffin), and the giant hat simply a tribute to him (after all the hat was a treasure to him, just like it was too Shanks and now to Luffy). This would explain the Hat, the “coffin”, and the Ice surrounding the whole room. Roger’s body could had been cryogenically frozen and preserved by the Marine/Celestial Dragons. Why?
This is pure speculation, but maybe they want to somehow bring him back to life with a Devil Fruit power and control him as a weapon. This would fit everything Doflamingo described perfectly, and also explain other stuff like the Marines interest on Bonney’s powers (Oda could easily tie age manipulation to a way to bring Roger back).
What do you think?
*Theory by Dracsxd