Padme and Anakin in their wedding outfits.
I have never heard of a halloween wedding before but this sounds so amazing I can’t even believe I hadn’t thought of it myself!!!
Good luck to you and I can’t wait to see the pictures!!! 
I say you dress up as a prince charming with a saber and epaulettes and all that such, and if she’s torn on wearing a really fancy dress but also being in costume, then any of the disney princess dresses would be a natural choice (personally I think Belle’s yellow dress from the ball room scene and the blue get-up that the Beast wore would be a perfect choice for the both of you and perfect for a costume wedding).
1890’s wedding gown, with you in Morning coat, top hat, etc.
You could do Gone With the Wind in the style of Carol Burnett.
Maybe the bride can dress up as the groom and vice-versa.
You are already going to be in costume…her as a Bride, You as a Groom…
Ran out of edit time…Just keep in mind that many of your older guests (frail Aunt Martha, Great-granny, the cousins on your Dad’s side) may not be as in to costumes as your friends are. And you are asking people who do not already have a costume at the ready to go beyond the expense of “pull out your dressy clothes and buy a wedding gift and get a babysitter”, so be mindful of that when you decide to require everyone to come in costume. While it sounds like a fun and fabulous idea for a party, remember that for decades to come you will be sharing your anniversary with Trick or Treat night for the kids, and as someone who deals with anniversary couples frequently, one thing many of them say is they regret sharing their anniversary with a holiday.
That being said, I consider angels to be non-denominational, so that might be lovely for your bride. And congratulations!
You realize you are making a mockery of the institution of marriage… you are laughing about the solemn vows that I and millions of others have taken in front of God and family. Makes my wedding and all others seem like nothing.
Oh wait… you are marrying a girl… so you’re straight… never mind, your good.
Why not do the traditional “wedding costumes”-- dress and tux-- for the ceremony, and then change into something more Halloween-y for the reception? That way she can still be pretty, you can still be dashing, and you can still make the most of the holiday. Plus, anticipation will be high at the reception for your arrival, with everyone wondering what you’re going to come as.
Frankenstein and Bride of Frankenstein?
Beauty and the Beast?
Kermit and Miss Piggy?
John Travolta and Olivia Newton John (in Grease garb)?
Romeo and Juliet?
Carrie and Freddy Krueger?
Bonnie and Clyde?
Both of you as Coneheads?
The Flintstones?
Princess Diana (with a tiara and also a train on the dress that goes the entire length of the venue) and you could go as any prince, but it might be nice to ride up on a horse.
Circus performers - of course, then you would need an elephant and a clown car…or just swing in on a trapeze?
Magicians…saw her in half and then say the vows?
My favorite - you come wheeled in on a hand cart as Hannibal Lecter, and she wears an FBI jacket over her wedding gown. Be sure to serve Chianti and fava beans for the reception.
I’m no girly girl, and the idea of a formal wedding gives me hives, but from the OP it sounds like your bride would actually really like the opportunity to be a bride. Her wedding will (hopefully) only happen once, but Halloween comes around every year. I’d ask her to really, really consider the traditional route before heading down to Spirit Halloween.
I also like the idea of a traditional ceremony with a costume party reception.
Jack Sparrow is so last decade. How about going as Daenerys Targaryen and Khal Drogo from Game of Thrones? The costumes aren’t enough though; you need the physiques to pull it off. But you’ve got seven months to bulk up.
I came in to suggest exactly this.
I agree with this. I think your bride has a choice to make, about whether she wants to be a traditional dressed-up bride, or whether she wants a Halloween wedding, but trying to find something in between is going to leave you unsatisfied.
Now that I’ve gotten that out of the way, I do have a suggestion. I have a friend who had a beautiful, traditional afternoon wedding, with a not-too-long cake and champagne reception. Then, that night, his fraternity threw them a party that was “formal cross-dress.” (It was absolutely hysterical, with a room full of bored-looking identically dressed women, and men running up to each other saying, “Where did you get that dress? I love it!” I helped the groom alter a thrift-store wedding dress to fit him, and the bride rented tails.) I think a traditional daytime ceremony and reception that your great-aunt Sadie will feel comfortable with, plus an evening party aimed at the younger set where you can go all-out with costumes, will come closest to satisfying everyone.
ETA: Oh, and congratulations and best wishes to you both!
Hence offering the admiral’s coat alternative, which always looks sharp. The Dread Pirate Roberts look is timeless as well.
If you do Westly and Buttercup, you’d need an Impressive Clergyman to perform the mawwadge.
If you really go all out, there might be a photographer and videographer who would do it for free, or at a great discount for their portfolio.
Congratulations!
I agree with the suggestions that you get married in traditional wedding clothes and then wear costumes at the reception.
That said, I went to a Steam punk wedding a couple of years back that was pretty amazing. The bride’s outfit looked something like this one.
(Thanks for the link, missred.)
How about a simple white dress with a little bit of fake blood on it and a katana for a Kill Bill bride costume. And, you can just dress as Clint Eastwood from a spaghetti western. It’s not quite from the same movie. But, they are both movie costumes, so that’s the thematic match.
Well, there’s always James Bond…
Y’know, I’m going to have to recommend against this one, the way things turned out…