What: When a creeper is on fire it should start its explosion sequence. This could include standing in a fire block, being attacked with a flame or fire aspect weapon, standing in lava. I don't think that magma blocks would detonate the creeper since they don't light the creeper on fire. Also charged creepers should be immune to this effect, otherwise when a creeper is stuck by lightning, the charged creeper would be on fire and detonate, making them unobtainable.
Why: Creepers detonate when they are right-clicked on with flint and steel. TNT explodes when it is lit on fire. It makes since that a creeper would also explode when it is on fire. This could be used as an effective way of fighting them, or making them explode in specific locations.
How: When the creeper starts taking fire damage it's fuse is lit.