Left 4 Dead 2 Message Board  

There are already mods to put the old maps in the new game with the new survivors. You can actually just transfer the map files over, and it should work just fine.

8v8 will never be a reality, according to Faliszek. He did hint that 8 player co-op might be a possibility though.

It would be a neat option to have the original survivors in the new maps, as there are already mods for this, but an official VALVe option would make things run smoother.