New world generator, that creates a landscape with a lot of pumpkins, webs and all kind of scary looking stuff. Or maybe even some rare chests with goodies?
Every player gets a pumpkin on their head (only one time!), But it would be a lit pumpkin, otherwise it would be too.. normal 
A nether gate brings you not to the nether, but to the world I mentioned above.
If 1.9 comes out on time, snowgolems will spawn naturally.
Random graves will generate on new land. (just for looks)
New towns, will generate with webs and pumpkins.

Jack-o-laterns aswell as pumkins.
Constant Night.
Custom Portals - netherrack portals? As opposed to obby ones.
Fire everywhere.
Haunted houses. 
A trick or treat system! - Cakes and stuff. 

Account that is premium with a skin:
 - Grim_Reaper