>>M.2 is a stupid form factor
We will eventually see those moved to the back (Biostar already does this).
Case and motherboard makers need to agree where to put access holes.
Can't even get at backplate of CPU as it is now, is kind of rediculous...
It doesn't matter. I think the trace path layout is going to have to be spread further across the board than U.2 would be. You still have to contend with heat issues as actively cooling an M.2 drive in almost any location is potentially problematic. M.2 is great because it's flat. It's nice for mobile applications but makes very little sense on the desktop. U.2 is a superior form factor in my opinon.
However, that's straying off topic a bit.