It would be great to have more dynamic sounds for killing (right now it's 3, having like 5 would be great) and other common achievements like: head shot, pizza delivery, knifing, max down,... and maybe something else. This will make it more various for people that doesn't make a lot of kill-streaks.
Also it would be nice to have single proximity mine kill voice before the 3 kill streak "present".
And new achievement for inactivity, like if you don't kill anyone for a long time.