I played on a server a while ago, I think it was some sort of zombie apocalypse type thing.
The zombies could RUN after you

I think that would be a GREAT feature to add to this plugin.
I would also suggest that have it configurable as to which level they can run
Example, ONLY above sea level (i think having them chase you in caves would be too crazy)
But if it was configurable ... the server admin has all the choice how they want their zombies to work.

I am loving this plugin on my server, I just feel that would add a little more difficulty

@towncraftserver I actually already used this plugin mixed with zombie apocalypse. Heck it might of been on my server you were playing on who knows.
@RoboMWM if you want to see how I incorporated everything into my server I would be happy to show you. It has taken me years to perfect it and balance it to make it work. I sort of built a very large plugin on top of everything to make an entire new game mode I call TotalWars.
More info you can find here https://github.com/Silverbane/MinecraftImpossible

Just a suggestion.
The slow down thing is only very short term... plus its too easy to get away from zombies, NOW them running after you, HELL... puts a fire under your butt to make you run faster.
I would LOVE to see this as a feature in EHM :)

Well, zombies do slow you down, so it is already giving this effect to a degree.