Great, well grounded theories, I think it's safe to say.
While I was reading this, and I came to the part where Luffy will require a motivation to fight the World Government after discovering One Piece, I couldn't help but think of Doflamingo and his 'New Era,' where the dreams of pirates is no longer the central theme of the Great Age of Piracy. We know that during the Marineford arc, Doflamingo was given orders from someone higher than Sengoku to murder Moria. If Doflamingo is receiving orders from this high ranking member of the World Government, perhaps this individual is also the proprietor of this 'New Era,' in which the World Government has enough power to directly restrict the freedoms of all inhabitants of the world, and only the strong will be able to survive in it. Even Doflamingo's powers allude to this. He is a puppet or a string man, able to create strings to control or dismember another person's body. If he is not the one manipulating world events to create his New Era in which freedom is incredibly restricted, where the dreams of pirates are longer important, then he is a waste of an allusion and a character. However, he does wish to maintain his own personal freedom, as he stated to the Marine officer he was reporting to. If he begins to dislike the agreement he has with the marines, if he can no longer maintain his freedom while also manipulating the world as he is currently doing, he will leave the Shichibukai.
With Doflamingo and the high ranking World Government official attempting to create this New Era that is so contrast to Luffy's ideals, I can easily see where Luffy is going to obtain his motivation to overthrow the World Government from.
(Sorry for any spelling errors or incoherence. I'm trying to become more eloquent with my writing, it's not going very well however.)