in my opinion the longest prime the m system needs is a portrait lens... 85mm on 1.6x crop is into an old fashioned 135mm, too long for classic portraiture perspective, too short for sports, nature.
A 60-70mm would be great, along the pentax lines, so a compact pancake, or maybe even a collapsable?
the caneras drive and af is up to portraiture, folk wanting sports or nature are currently better served elsewhere.
Of course a revamped f1.8 85mm would be great in ef mount, so long as we are in the model of the 50mm f1.8 revamp - not vastly more expensive than the original... rather than the 28 & 35 is revamps.. very nice lenses, but double / triple the price of the originals.
I guess when I think of the m I can't help think of the old Minolta / Leica CL, or himatics... compact form, keep it simple.
A conventional 85mm would be a big lens for a pocket camera.
Again just my opinion, but the M and its systemic parts need to be 1. Unique 2. Compact 3. Affordable
The M is a great gateway to the EOS system for new users, and a great back up to exidting EOS users.. I don't see the logic, from a marketing perspective or user perspective of recreating the EOS system in minature...
Keep it compact. Keep it affordable. If folks need to specialise beyond this then let them adapt the EF system, something which no other mirrorless camera has access to, and which is an underused selling point.
Canon should be bundling an EF/ Ef-s converter inevery m box.