So
.. you want a telephoto for the M series with constant aperture or the constant aperture isnt very important for your use? I usually use the M50 in manual mode, so a constant aperture is very important to me
I'm looking for fast primes in the small form factor. The largest entrance pupil so far in the small form factor is about 40mm with a 55mm filter. Assuming this maximum for the small form factor, some possible lenses would be about 40/1.0, 56/1.4, 80/2, 112/2.8, 160/4, and 224/5.6. Depends on where you draw the line for fast primes - I'd be interested in primes up to 2.8 in the small form factor.
While not impossible, I doubt Canon will create another lens series for EF-M with a larger form factor any time soon. So that leaves adapted EF lenses which I accept, ergo the 70-300L with an entrance pupil of 55mm and 67mm filters in my M kit and the occasional use of the EF 85, 100, and 135. A series of lenses in this form factor (55mm entrance pupil) could yield 55/1.0, 77/1.4, 110/2.0, 154/2.8, 220/4.0, and 308/5.6, but might not gain me a lot over my adapted EF glass. I view larger entrance pupils on native EF-M glass quite unlikely. Canon would prefer you to buy the R series.