Though I won't buy it without seeing how it plays first(No...I did not buy Ghosts), I would change the following:
New and varying in size maps. Don't make them all small or huge.
Toughness should not exist in any form for any weapon type. Neither should any perk that allows sound whoring. For that matter, take out stock and quickdraw in all forms as well (see weapons).
C4 should be planted, not thrown.
Make classes of weapons actually matter. Don't up RPM on guns that require more bullets to kill just to make every weapon almost the same. There should be pros and cons to each weapon class and variance among each weapon in that class. Liked the idea of Marksman class from Ghosts but not sure on implementation.
Dedicated Servers only. There are too many reasons they are better than the manipulation, inconsistency, and general crap that is the last system of host based/peer/listen/whatever system they used. I won't go into lag comp because I know the reasons to have it and dedicated servers would address a lot of the problems they had with it.
The usual. Better placement based on location of enemies and more possible randomness. It could even kick in at different levels based on location of enemies.
Fix where bullets/tracers/etc are fired from to prevent head glitching while keeping cover.
Just a few off the top of my head.