We can see that the profile suggests a hatchback with the long, slowly sloping roofline and windows that extend past the rear door. The lower window line sweeps up aggressively toward the rear, a little reminiscent a Mazda3. The nose also looks relatively long, potentially giving the hatch rear-drive proportions. Not that the shape would be necessary with electric motors and batteries, but Rivian has shown that it still likes using time-tested design in its vehicles with the R1T and R1S, two traditionally boxy, aggressive looking trucks.
According to our spy photographer and his designer source, we shouldn't get too carried away with design speculation from this one prototype. Apparently these design bucks are used for testing sight lines, seating arrangement, ergonomics and such. As a result, the exterior design is far from finished, since most of it won't be seen from the inside. But the window shape does suggest a somewhat curvy hatch, and our photographer notes that the fenders could be similar to production, as they could affect visibility if they're as wide as they appear to be.
We'll be watching the development of this car closely. Based on the R1T and R1S, this hatchback will surely have all-wheel-drive from two electric motors, lots of power and a large amount of ground clearance. And it will probably be relatively pricey.