The idea of the frozen straw hat being a symbol, related to the Will of D or not, is pretty compelling. There doesn't seem to be much other way of coming up with a reasonable explanation for it. But it doesn't seem to line up very well with what Doflamingo's hinted about Marijoa's national treasure. In particular, when he first mentions it, he suggests that its existence be enough to shake the world, and that with the Op Op Fruit, he could make use of the treasure and rule the world. He specifically mentions the personality transplant and the ability to grant eternal youth. It's hard to reconcile this with some symbol of the past. And it is hard to reconcile it with a big straw hat in general.
Maybe the straw hat is a red herring? Something significant, but not the treasure which Doflamingo's talking about. This makes the framing of its reveal seem odd, but hey, the whole thing is odd. As long as the treasure is also in that room, it would still make some sense.
This is all based on my interpretation of a poor translation of one comment Doflamingo made, but bearing that in mind, …
What could such a treasure be? Well, it would make some sense if either the personality transplant or the eternal youth operation were needed to leverage it. So either
- it makes you old, somehow similar to the drugs on Fishman Island, but if you have eternal youth, you can just use it indefinitely, or
- it's a person, and it's their body (preserved in that icy room) that's useful/powerful, hence the personality transplant lets you occupy it, or
- it's a person, and it's their knowledge or skills that are useful, but they're on the verge of dying from old age - you could give them eternal youth, or put them in another body, or
- something I haven't thought of.
Some of these can be achieved in other ways though. Leveraging a powerful body can kind of be done using Moria's powers - although if for some reason you need/want it to really be you who has the body, then that doesn't work. The eternal youth thing can probably be done using Bonney's powers. It's interesting that Akainu went to retrieve Bonney from Blackbeard, and was concerned at her escaping the government… but the Five Elders mentioned her in a totally offhand way (that one of the >100mil rookies had already been taken down by Blackbeard), so probably that's more personal (aside from the fact that it's unlikely that the Fleet Admiral knows the Celestial Dragons' secrets - Akainu was very much kept in the dark during the Dressrosa arc). In any case, Corazon implies that Doflamingo's plan was to have Corazon give him immortality, but he might not have known...
Putting that aside, given that Doflamingo was looking for the Op Op Fruit, and at the same time looking to take over his family's erstwhile kingdom, and is kind of a planner, what was the plan? Okay, so the explanation on the face of it is that he wanted the Op Op Fruit to become immortal, he wanted to take the kingdom because he saw himself as a god-king thrown out from heaven and since they were never going to let him back in, he was going to take the next best thing - the Donquixote kingdom of before the age of World Government. He uses his knowledge of the secret treasure to get Celestial Dragon-like privileges, and becomes a Warlord also, no longer a wanted man. At some point, I guess fairly recently, he begins to manufacture Smiles using the SAD developed by Caesar. The so-called factory in which the Smiles are made is actually an orchard, worked by Dressrosa's resident (or neighbouring, I guess) horticultural specialists, the Tontatta. This is interesting to me, because we learn that in the past, the ancient Donquixote kings had also enslaved the Tontatta... What if the old Donquixote kings created the Devil Fruit, leveraging their slaves' area of expertise, and this was the weapon that allowed the alliance of twenty kings to overturn the enigmatic lost kingdom? Maybe Doflamingo's plan was of the form Op Op Fruit + Tontatta + Marijoa's treasure = Devil Fruit Factory, and, the loss of the Op Op Fruit having thwarted him, he found another way, eventually.
Maybe, under that giant straw hat, is a frozen ancient Devil Fruit gardener :ninja:
Silliness aside, if the treasure were related to the source of the Devil Fruit, it would be more of a shocking reveal to the world, for whom the Devil Fruit's origins are kind of mysterious (as far as we know). Something organic would explain the freezer, and could be supported by Doflamingo's choice of phrasing (that power rots away). Plus, it makes sense for the origin of the Devil Fruit and their mechanisms to be a big plot point and somehow wrapped up in World Government/history, because it's had its sporadic hints here and there (like the detail in showing Smiley's power going into a nearby fruit, despite the fact that it wasn't re-used (so far)), and because Blackbeard seems to have some idea what's going on on this front. There's also the possible nice thematic element that if the World Government used Devil Fruits - which endow people with powers in exchange for becoming the natural enemies of the sea - it would put them in nice opposition to the lost kingdom/the poneglyph authors, at least based on the two weapons we know about (both being strongly related to the sea in operation, and one being from a city of water sinking into the sea, and the other from 10km under the sea). Add on that World Government, after forming, moved themselves as far away from the sea as land gets...
Thoughts ?