I don't have much experience playing D&D and the like but I enjoy reading the manuals and thinking through cool ideas. One I want to try is the Positive-Energy Charged Creature
An undead with fast healing 5 and a 5 feet aura that heals living creatures and damages undead for 1d6 per turn. In addition, not only do all its negative energy damaging powers become positive energy (sadly though energy drain does flip to positive, it doesn't give temp levels), but its natural attacks become positive energy too so you get healed by getting hit. There's also overhealing, so all the excess energy gets converted into an equal amount of temp health.
It gets better, an undead killed by a positive energy undead gets transformed into a positive energy undead with full hitpoints, though it is staggered for up to 4 rounds and not under the control of its maker.
This seems like the perfect undead killing machine doesn't it? Except for one thing, Overcharge. If you get more temp health than your maximum HP, you explode in a burst of positive energy. There's no save involved, the body just automatically disintegrates and sends out a burst dealing 1d6+1d6*(temp health at time of death/10). The burst is positive energy so you can actually trigger a cascade effect and wipe out a whole slew of creatures in a single turn.
They still have a weakness to positive energy and acquired an immunity to negative energy as part of the process so they aren't really good in groups but you could still have a lot of fun with these especially if the players fail the check and don't recognize the danger. Maybe NPC's have been using them in an infirmary and don't realize the danger because only the injured pass by and they never get enough temp health to explode.
I'd like to see them incorporated into a castle. Place them in cages in 10 ft intervals. You could either use it offensively, buffing your assaulting units as they disembark from a fortification, or defensively; any unit with hit points less than the number of zombies gets vaporized before it can reach the end of the corridor.