This is a post has been made for the creation of new single medals so no you cannot have Killing spree, Be the bullet etcetera. Here are my Ideas though.
Backing Up. (Splatter an enemy with a vehicle while driving backwards)
Soaring High: Survive what would have been a deadly fall by assassinating an enemy player.
Basically turning that Reach achievement into a medal.
Hot potato: Kill an enemy with their own grenade.
Collateral: Kill an enemy with a vehicle you are no longer in.
Cone’d: Kill someone with a cone.
Seriously I see no reason this one shouldn’t return, other than a lack of cones. I know it wasn’t officially a medal, but it was tracked on B.Net.
Denied: Kill an enemy this close to capturing the flag.
Tried to make them at least remotely unlikely to happen so that it didn’t create a constant stream of medals. That’d get boring after a while.
Odd bomber: Kill an enemy with incoming ordnance.
Wolfkill: Assassinate an enemy while jumping out of sprint.
Thermopylae: Lose the game with the medal ‘unfriggenbelievable’.
Savagery: Kill two or more enemies with one swing of the Gravity Hammer.
Forklifts never die: Kill an enemy by blowing up a forklift.
Insurance: Kill the enemy hijacking your vehicle
Buddy Bump: Cause an enemy to fall to their doom by simply colliding with them. (Via vehicle or sprint.)
Waste Not: Get a triple kill or higher with a single shot.
Top Gun: Kill an enemy while airborne. (Jetpacking, grav lift/cannon, vehicle; not Banshee)
Some could also pass for achievements, but hey! I did what I could. 
Not sure what it would be called, I don’t have any ideas, but a medal for grabbing the flag and capturing it without dropping it.
Neglection!
When you pick up a weapon and carry it through the game without dropping it or depleting its ammo.
High-abusa: Kill an enemy from an elevated position.
The medal should have an image of an eagle.