9,000BC Halloween Zombie Land Edition Needs your Ideas

+1, you could have it unlockable via an easter egg

 

Getting all the achievements in feint or getting a the hardest one.

 

will this be an update to the original, or will i have to buy this game too? Like a toggle between modes?

edit: saw someone suggested, but still. A toggle would be best. Because, yes, it would be annoying, like stated, to play Halloween mode all year.