We mainly get a look at the taillights. They appear to use an LED light strip for primary lighting. They also stretch forward and around the rear fenders quite far. This matches up with the headlights, which also reach back toward the wheel arches.
We can also tell that this is a garden-variety 8 Series, rather than the M version we saw earlier. This is because it has much smaller wheels, tires and brakes. It also has the exhaust outlet fully integrated into the rear bumper, like the coupe. On the M version we've seen, the exhaust uses more traditional round tips.
We currently expect the BMW 8 Series to arrive for 2020, though given how far along this test mule seems to be, there is a possibility it could come a year sooner. Either way, we still have a bit of a wait before BMW officially reveals its 8 Series flagship coupe and convertible.