I'm very interested to see how OpenCV 4 and YOLOv3 object detection will run on the new RPi. At the current trajectory I will have to upgrade to a Jetson Nano to get hardware acceleration (CUDA), but resorting to specialty hardware seems like cheating.
They also make a great prototyping platform for IoT projects. I've built 802.11.4 (Zigbee) mesh radio networks for passing small messages across neighborhood distances.
Previously I used an RPi to run an Airplay Bridge to my Sonos speakers. This has since been supplanted by AirPlay 2, which Sonos supports.