The idea has been brought up before, and I still like the idea of this specimen.
How about this for an exploding ZED:
Behaviour:
* It runs quite fast
* Charges when getting close to players (Similar to Gorefasts)
* It is quite weak in health
* If it does its regular suicidal attack, it explodes instantly
* If it is killed normally (bodyshots/headshots), it will explode after a little while (giving Berserkers (and others) some chance to run away), having a "bubbling/twitching" animation until it explodes
* Can also be killed if enough damage (low amount required) is done to its explosive body parts, but this causes it to explode instantly
* Its explosives obviously deals explosive damage (giving the Demolitions perk some reason for his resistance other than against selfdamage and Patty-rocket)
* Its explosion can hurt other ZEDs, detonates other nearby explosives and deals quite a lot of damage.
* As rare as Scrakes, but appears on wave 4
Appearance:
* Can be very loud with crazy screams (hard to miss its proximity)
* Twitches a bit, a lot more so when charging
* Skinny and tall
* Its arms being the explosive parts (sorta similar to the suiciders in Dead Space?)
More ideas to that?