Let's assume that he becomes the star we think he can; what kind of contract would you like to see offered in his 3rd year? And would he want to stay here?
I have no idea where Bryce Harper wants to play in six years. From what I know of his background, he didn't seem to grow up a fan of a particular team, and the teams near his birthplace are mostly small market teams.
If he is like 99% of athletes, he will go wherever he gets the most money. The only exception is if he is risk-averse. The guys who sign long-term extensions during their first 6 years are guys who would rather take sure money today than wait for bigger money tomorrow with the chance of a career-ending injury and no money.
I have no idea what his risk preferences are, but if he is super confident in his abilities maybe that suggests he'll test the market. Who knows who will win the bidding war.