Mixer and formal themes are so overused because everyone recycles through the same 10 ideas: Jungle, 90's, Mathletes and Athletes, Tacky Tourists, etc. But there are so many more themes out there to choose from.
1. The Bachelorette
I promise there won't be as much drama as a real episode but have your sisters ask their date with a rose and the typical saying
"Will you accept this rose?"
2. Get A Clue
For this one you can either a) use a clue hunt to have your organization figure out where the party is or b) turn it into a murder mystery and the suspects are the party guests.
3. Blacklight/ Highlighter
4. Double Vision
Although you don't have to rob a bank to pay for this theme, you and your date just have to match.
5. Mad for Plaid
6. Flappers and Gangsters
Have your social chair invest in some water guns for a "real" brawl.
7. The Masters/ Caddyshack
8. Foam Party
the struggle with this one would be finding a location.
9. Chacos and Tacos
10. Drop it Like it's Yacht
11. Dynamic Duos
Anything from Dory & Marlin to Meredith & Christina to Boo & Sully.
12. Boots & Boxers
By far the comfiest Mixer ever.
13. Derby
14. Thrift Shop
15. Out of this World
Party On!