The capabilities of the viewfinder are a big unknown. The issue is that to accomodate people who wear eye-glasses and the range of available focal lengths, Leica has over time introduced 3 "fixes" to the basic design - different magnifications set in stone when the camera is made, a screw-in viewfinder magnifier and auxiliary finders on the hot shoe.
All of these are unsatisfactory from a cost and usability point of view. In the ideal world, there would be one viewfinder which would handle 15mm - 90mm (accepting that 135 is out), provide an exit pupil which suits eyeglass wearers and provides a variable magnification finder to suit the lens mounted, avoiding the need to choose viewfinder type at purchase and buy these slightly archaic accessories.
If they've gone for minimum change, there will simply be frames for 28/90, 35, 50/75 only and the viewfinder magnification will be 0.72 * 1.33 = 0.96, a bit like the R-D1 with more frame lines. In this "solution", you'd still need to use an aux finder for wider lenses.
If they've been brave, they could have gone for one or more of the following:
- a dual range viewfinder magnification to avoid the need to specify at purchase and use the viewfinder magnifier
- support down to the 21mm (28mm equivalent fov), with frame ambiguity resolved by using the zebra coding
- use of an LCD screen to project framelines and other viewfinder information (shutter speed/flash) into the viewfinder.
Can't see the 15mm being supported without an aux finder but if the 21/24 are not supported natively, a new aux finder will be required for the narrower angle of view.
The viewfinder is a key product differentiator for the M8 and Leica had the opportunity to introduce something really innovative instead of sticking doggedly to tweaking what has gone before. If ever the time was right to re-invent the M rangefinder concept, it was now.
I use the past tense because this has long been decided and my fear is that, like a rabbit caught in headlights, they've been scared of being sufficiently radical. Time will tell.