Immortality and the Treasure of Mariejois
I haven't seen much discussion about Doffy's mention of the Mariejois treasure since the giant straw hat was revealed, so I'm hoping to start some.
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From this exchange, we know Doflamingo needs the power of the Ope-Ope no Mi to "make use of" the treasure. Not access it, not obtain it, make use of it.
This scene supports the idea that Doflamingo wants to have the Immortality Operation performed on him, based on Rosinante's internal monologue interpreting Doflamingo's words.
And if we weren't already sure, this really drives home Doflamingo's intent. He wants the power of the Ope-Ope no Mi — he wants immortality.
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To bring us back to Doflamingo first mentioning the treasure of Mariejois (with a slightly different translation for good measure): Based on our previous findings, we can come to the conclusion that immortality is needed to use Mariejois' treasure.
Doflamingo's choice of words here feels very intentional: "power degrades quickly", "it rots away in no time at all"... Definitely seems to fall in line with the idea of mortal bodies degrading/rotting. I also get a somewhat morgue-like feel from the room Im enters, since it's a cold room with multiple alcoves in the wall — at the very least, the chilled air seems to imply preservation.
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Regarding the treasure itself — while it's not outright stated that the giant straw hat is the treasure, we're certainly led to believe it is since it's revealed after Doflamingo mentions the secret treasure of Mariejois again.
The treasure could even be the whole room, since there are other unlit alcoves — and Im does happen to be holding Luffy's wanted poster (and a second obscured poster) while viewing the straw hat.
What does immortality have to do with the treasure?