If Subaru had a gearbox capable... I would love to see a 300+hp H6 version of the Legacy and the Tribeca, and Forester... although perhaps turbo 4 for the Forester....
The H6 would be smoother, have more predictable response, and torquier at lower RPMs, before a turbo 4 would spool up, and allow a lower cruising RPM, and better cruising gas mileage.
Brembos, Bilsteins, SI-Drive calibration, VTD/DCCD calibration, maybe even active suspension damper adjustments, and either a manual gearbox, or paddle-shifters on all of them. I would love to see a PDK-style clutch-based automated gearbox on the upper end cars... to appeal to people who don't want three pedals, but with just as much performance and direct connection to the road, as a car that does have 3 pedals. Torque converters sap performance away when they aren't locked up, and generally hydraulic-shifting isn't as quick as electro-mechanical.
And I wouldn't mind if the next WRX/STI body were a 3-door coupe, and 5-door sedan (think smaller, sporty version of Audi A7, sleeker than Impreza 5-door/XV), with either 4 side doors, or two longer side doors.
...to separate it from Impreza's 4-door sedan, and 5-door hatch.