According to Motor Trend, the 2013 WRX, which will likely debut sometime in 2012, will go back to its original engine size. The new powerplant will reportedly be based on the engine found in the 2013 Subaru BRZ, a clean-sheet design for Subaru, with the addition of the expected turbocharger. That ought to bring power up from 200 horsepower to about 270.
Now we come to the question of what platform in which the engine will reside. While common sense would dictate that the latest version of the Impreza, which was recently released as a 2012 model, would get the WRX treatment, we've heard rumblings that Subaru would keep its current WRX shell around for a few more years while it continues development of the next chassis. Only time will tell.