Oh that sounds like too much fun! I see the tiers as a great opportunity for all sorts of ghost and goblins peeping through whether separated or stacked.
My first thought was a stacked cake mimicking a tall haunted building. But even separated tiers with little haunted scenes between each layer... like a graveyard on the bottom layer and flying ghoulies on the upper layers. Pumpkins, squashes and vines as bottom borders. Nice crisp edges, no borders, on the tops of the cakes.
For a topper, Frankie and his bride, Dracula and a victim, zombies, or two skeletons.. the possibilities are endless.
I don't know if you can open this picture because it's a membership site (but you can view the site free for thirty days if you go ahead and register). Here is a picture of a cake described like sgtx80 suggested.
http://www.ladycakes.com/attach/133684-karen-for-web.jpg
It's a bit blurry. It is a 3 tiered (bottom two stacked, top separated) with oak leaves embossed (white on white) on the sides of the cake. Fall colored leaves cascading down the sides. Bride and groom (complete with top hat) pumpkins on top.
Good luck. I can't wait to see what you decide to do!
does it have to be sacry? unfortunately i don't have a pic of this, but i did a 3 tier cake a few years ago with a fall/halloween theme and it was REALLY cute! instead of putting a border around the bottom i alternated pumpkins, acorns, indian corn, fall leaves (all made of b.c. icing), the top borders were rings of fall colored leaves and the center of the large cake was a pumpkin patch with ghosts peeking out. i know i'll be kicking myself for years to come over not snapping a quick pic of it but live and learn! 
good luck!!
Well its been a while.I did manage to make the wedding cake. It was a four layer dark orange with spider webs (black) the topper was a skeleton couple sitting on a bench I didnt get a photo and it wasnt one of my best I came down with a bad cough/fever which I still have I sure didnt feel like making a cake. Thanks so much for all of your help its great to ask for help and get so many ideas. The bride decided on this one, Thanks again 
Well yuck on the cold and the no pictures!!! I've worked with the flu and it sucks. I wore a mask and gloves and washed my hands a lot (all the way to the top of the arm) to keep from spreading germs. It was horrible.
Make sure you always get pictures, even the bad ones. It really does matter. I've gone back to look at cakes I made a long time ago and can see the improvement. It's a great guide for you. Even if you don't share them with anyone else.
Feel better!!
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