I think Dany is going to arrive back on Westeros and find the country pounded by all the wars that they don't even bother to fight back. She's going to be ready for war and there's going to be little left after The Mad Queen burns Kings Landing and the North has been reclaimed by the Starks.
With Tommen dead, and presumably Margery, who has the claim to the throne? Cersei will probably be dead by the time Dany arrives too, if you believe in the valonqar theory.
I think Westeros is not going to be the prize that Dany thinks it is.