The Pico3D engine, designed for the PicoSystem console from Pimoroni, is an open world game engine designed for microcontrollers. Binaries and source are now available on GitHub: https://github.com/bernhardstrobl/Pico3D.
Note that the source code is currently just a code dump without much help in the way of getting started. A tutorial will be available in the coming weeks to help with that :).