It looks like Canon is developing its own speedbooster for EF lenses on the EF-M mount.
Canon News uncovered a patent last week that showed Canon was looking at a new EOS M adapter, and again this week, a new patent showing the concept in more detail has emerged.
Japan Patent Application 2018-189864:
This patent application is for a converter that takes EF lenses and reduces the focal length and decreases the aperture all at once.
The example in this patent applications shows a 50mm f/1.4 lens (full frame) with a back focus of 39.80 (EF full frame) and converts the lens to a 40mm f/1.2 with an image height of 13.66 and an 18mm back focus, which would work for EOS M cameras.
If Canon is indeed going to keep the EOS M line around, developing a speedbooster for the system would be quite a nice addition to the lineup.