For me, I'd say an entire overhaul of character mods and player mechanic effects because there is only one mod players go to, which has the least draw back of any. I'm talking about rewriting the mechanic in general because what's there isn't giving any opportunities for varied effects.
Specifically, Fragile has to have a certain mobility advantage that actually significantly effects combat performance because extra speed does nothing when base speed lets players dodge bullets fast enough, Flak Jacket letting you actually tank explosives (If CoD:BO Flak Jacket let's you eat a car explosion, the f*ks the matter with doing it in APB, then?), and Kevlar's protection being different than just health increase to balance it with all weapons instead of some, all while the downsides to all of those are way too crippling, except Flak, where it's the low reduction that's the problem. The basic mechanics, and probably the whole coding of characters, has to be overhauled, though, which is a much bigger problem than this issue here. The fact still stands that character mod variety is non-existent and a lie, which a new player will find out soon enough.
Making it harder to gain threat and easier to lose threat would definitely bring me back full time, though.