I've been watching the Subaru rumor mill for over 10 years. More often than not, dealer gossip and forum speculation turn out to be just that...gossip and speculation. Until Subaru calls a press conference, we really don't know what they're bringing.
They may incrementally improve mpgs on a successive model...but if fuel efficiency is what you're about, an AWD sports sedan is never going to be optimal. The 6th speed has been asked for in the WRX for a long time, (actually, not 6 gears so much as the strength of Subaru's STI gearbox)...it's possible, but it's reasonable to assume that they could also leave it out to help keep the cost of the car down.
There's always going to be "the next model". I'd get closer to the time to buy, and then go drive the car. If it puts a smile on your face, and works with your budget and lifestyle, then there you go.
Welcome to ClubWRX.