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.
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!