I'd like to bring this topic back from the dead. So sue me for the Necro
Yeah yeah we already know what the 2018 WRX STI is like as of this post, but there are more new bits we need to consider now.
The new Subaru Ascent 7-Passenger vehicle coming out in late 2017/early 2018 will be featuring a new FA24DIT Boxer Engine. No numbers on this, but this will be the first vehicle Subaru has built with this new engine in it. It's perfect timing if we look at our Crystal Ball to predict Subaru's future using past models and formulas they have used. In 2013, when the 4th Generation 2014 Subaru Forester was released, the "XT" version featured the new FA20 Turbocharged Boxer Engine we'd later find the following year with the 2015 WRX (released in the spring of 2014). The STI hasn't seen a new engine in ages and it's what enthusiasts have been asking for ages. The new Ascent's engine will answer that call in 2019 when we get the 2020 WRX STI built on the Subaru Global Platform (same thing the new Impreza is currently on).
I'd be surprised if Subaru gets back to the WRC. The Impreza is too big to go toe-to-toe with the smaller hot hatches we see dominating that circuit currently. I mean, look at how much the new WRX STI platform has struggled even in Global Rallycross. Subaru would need to release a smaller car in the US, smaller than the Impreza, to be competitive.
We also need to admit that, at least for this current generation, the Hatchback isn't coming back. Europe, Australia, and Japan get the Levorg (which is a wagon instead of a hatch) and the US audience just doesn't like wagons I guess... which is why the Outback doesn't sell (obvious sarcasm). Our best chance for getting a hatchback WRX or WRX STI back again is with this new global platform again. So, that'd be with a 2020 model (released in 2019).