Back on topic. I have no facts to back any of this up but my prediction for a more powerful BRZ is this. Won't be out for at least a year after the redesigned WRX hits the market. That should be the next big performance product release for Subaru over the next year or two. The WRX will generate press, so would a BRZ STI. If they released them at the same time they'd be competing with themselves for attention.
For power the FA20DIT makes the most sense. Its an off the shelf motor so minimal engineering expenses. It gets great gas mileage for its power levels. The torque comes on nice and low and peak tq is flat for a few thousand rpm so should be pretty responsive in a performance application. Makes a lot more sense than re-engineering the N/A FA20 to make more power since there isn't a lot to be gained without drastic work. An N/A H6 would be cool but the current H6 gets horrible mileage compared to the turbo FA20.