In the delightful response everyone has given, I'd like to put my two cents into what I would like to see with som DLC.
And I agree, lots more new weapons. One of the few things that made the Single Player Story Mode so fun, whether you were playing seriously or for fun - was the variety of weapons.
Preparing a reasonable weapon class to suit your next mission was always fun.
And to be honest, there needs to be more added to that.
I propose this idea, create a new DLC resolved around a great number of brand new weapons. Nothing too serious, just a list of about 10 new weapons available for Multiplayer and Single Player.
Silenced M4 Carbine (There's not enough stealthy weapons)
to name a few