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From my experience, AI is really cheap about granting Kingdom-level titles, although that might be for good reason. So don't count on it.
But overall culture, religion, liege opinion towards you and your opinion towards liege all influence whether he's gonna grant you any titles or transfer vassals to you.
He did give it to me eventually when I became his friend.
What is a bit annoying is that the grand duchy of Florence does not exist de jure in the beginning (just the kingdom of Italy), so you have to create it by not becoming Kaiser for 100 years.