A few things that are likely certain on the next m3:
It will be FI.
It will have a faster 0-60 and 1/4 mile time than the e9x.
It will have more HP and torque.
It will have a faster ring time by probably 10 sec or so.
It will "feel" much faster than the e9x around town.
It will likely weigh the same or slightly more.
It will, with time as the e9x becomes more dated, look better.
It will have better mpg than e9x.
It will cost more.
It will not be faster than or as fast as a GTR.
It will not sound as good as a e9x
What we don't know:
Which engine layout.
If it will be as reliable as a e9x.
If the leather will hold up better.
If it will be as fun to drive, though I would guess yes.
I think that about sums it up.