maybe you can tie in rocket surgery into your storyline, "rocket surgery gone wrong"
I'll try to keep this on topic. I was thinking last night about potential baddies, and I came up with Super Mutants, and Security Drones:
Basically Super Mutants would be very muscular behemoth sized mutants, twice the size of a regular player (sort of resembling The Hulk). They were created in almost the same fashion as the Security Drones; humans that were genetically modified to serve as a shell for the drones but managed to break free before any of the robotics implants were put in place. Due to blindness (broke out before ocular implants were installed) it will attack a player only if it comes within a certain radius (as it now uses hearing to detect enemies). Once a player has entered this radius it would let out a piercing roar and charge a player in a blind rage, smashing away any obstacle in its path (as in broken cars, fire hydrants, etc). It would have two types of attacks, swinging its arms (left to right, right to left), or pounding the ground (with a stun radius for any unfortunate player who gets too close); all of which would be devastating if hit. It would move slightly faster than a normal player. It would be passive to zombies.
Security Drones would be rogue cyborgs left over from when various private security agencies tried to combat the zombie population. Unlike the Super Mutants, cyborgs went through full robotic integration, but once the drones became self aware, they betrayed their creators and now wander around the wastes, killing whatever they please. The drones would be a bit larger than the Super Mutant, and would have sort of the same frame. Drones would have ballistic plating covering most of their body (really they'd basically be an upgrade of the Super Mutants). Their method of attack would be from a missle launcher/chain gun which is fully integrated into the Drone's left arm, the right arm would be free. If player gets too close it will swing in the left to right fashion, like the mutants. Drones would move slower than the Super Mutants. If possible they will attack both player and zombies (but not Super Mutants). If a player kills one of these, they will find ammo, and possibly gun upgrades.