We are not wind tunnel specialists, but the difference in fuel consumption between the current Prius body and a wagon can't be that big. Why would you even bother? Because in Europe, cars like the Octavia Combi, Golf Variant and Mercedes 5 Series Tourer are best sellers.
Toyota must know this already since it sells a vehicle that fits the description. We are talking about the Auris, which is one of the few vehicles that's available as a wagon with three types of engine: gasoline, diesel, and hybrid. More and more people are buying the hybrid model to the point where Toyota stopped developing diesel engines and only offers a few it borrows from BMW.
In some ways, the Prius Wagon would be counter-intuitive. A wagon is what you buy if you make several long trips that require a big trunk and the hybrid gets its best miles around town. But anybody who's owned a Prius can tell you it can surprise on the highway as well.
There are also several features that the Prius offers and the Auris doesn't. For example, the new Prime model is available with a huge tablet-like screen like the Tesla Model 3. You can also order an electric motor for the back axle to make it AWD. The suspension is also much more advanced, and we could go on for quite a while.