I love the challenge your mod adds to Minecraft. Thank you. Zombies aren't quite the pushovers they used to be.

I've seen the feature I have in mind done in the "Extra Hard Mode" bukkit plugin and after trying your mod I thought it would be a perfect fit:

Unless it's on fire when it dies a zombie will respawn at the point of it's death after a very short delay.

I thought this would compliment the system you already have in place to add even more challenge. Of course, not everyone might like it so perhaps a config option to disable it would be good, too.