It should be no surprise that a full WRX redesign is a few years off. Although the fourth generation Impreza was released in 2011, the current WRX has only been on sale since the 2015 model year. Given an average five-year cycle, we can most likely expect a redesign for 2020. It's not clear how extensive the refresh will be or if the STI will finally drop the ancient EJ-series 2.5-liter turbocharged flat four engine for the more modern FA-series 2.0-liter turbocharged flat four engine found in the regular WRX.
Rumors have been swirling for years of what else a full-redesign may encompass. The current model was rumored to have an electric turbocharger that obviously never appeared. There have been other rumors of a switch to a hybrid powertrain. While that may work when the WRX finally uses the updated Impreza platform, don't expect to see hybrid power in the refreshed model. Subaru says the time it would take to engineer such a system would be too great to have it available for 2018. The new Impreza's platform is designed to fit a hybrid system, so don't rule out the possibility of a WRX hybrid.