For each follower goal that is reached, more prizes are unlocked. As of right now, the only physical prizes are a stationary set and a T-shirt, and they are only available to those in Japan. However, digital prizes are available for everyone and include twitter icons and desktop wallpapers, which you can check out below.
The next set of prizes has been revealed to be a set of autographed illustrations, and will be unlocked when the Final Fantasy XV Japanese Twitter reaches 50,000 total followers.
Final Fantasy XV is scheduled to be released for Xbox One and PlayStation 4 on September 30th.