i'm want a WoW wedding too!!! it's still like 2 years away, but i've been brainstorming. here's a few ideas that i already came up with.
invitations that look like quest logs. these would be pretty easy to make. you could set up quest objectives (witness the marriage of __ and __) and quest rewards (a night of food and dancing). i found out the font they use in the game is called morpheus, and it's available for free here: http://www.urbanfonts.com/fonts/Morpheus.htm. you could use it wherever else you need a fancy font.
on the place cards, you can use character names for your guests who also play WoW.
instead of table numbers, use locations in the game, or realm names.
for the cake topper, make little sculptures of your character and his character out of polymer clay. i play horde, so i also want the cake to have the horde logo on it all over in frosting. i've seen this done before with white-on-white writing and it looked nice. it adds some detail without being too loud or tacky. you could also do this with common in-game phrases (for the horde/alliance, pwn, n00b, l2p...).
my wedding colors are going to be black and red, like the horde colors. blue and gold would look nice too, if you play alliance. i want my dress to look a bit medieval too. i'll probably have my mom make it from a halloween costume pattern (she's the grand master tailor in rl lol).
for favors, i'm doing red and black m&ms in little pouches, like bags of loot. i wanted to do a little card stock chest, but i think that would be too much work on top of everything else. they sell the little bags pretty much everywhere you would buy favor stuff. maybe add a little tag to it that looks like gear stats?
if you're serving wine or champagne in bottles on the tables, you could make your own label with stats like buff food has. like +30 spirit, or gives the user the ability to dance the night away.
for food (or even just appetizers), you could see if the chef can recreate some of the foods from the game. like mini sliders could be warp burgers, or a spicy salmon dish could be labeled as firecracker salmon.
wow, i can't believe how much i just typed lol. good luck and keep us posted on what you come up with!