My daughter recently went to a 21st. The girl's name was Laura, and she said everyone had to be in fancy dress connected with the letter L. My daughter dressed as Leonard from the Ninja Turtles.
How about a real grown up princess party...lovely pink dress ( stylish and beautiful )..all little girls want to be princesses why not still do it as an 18 year old .... have a chocolate fountain with strawberries to dip..cute little sandwiches and butterfly cakes.....you may laugh but really open your mind and think of all possibilities
Colour theme eg pink with people wearing pink plus rose wine, strawberry mousse, that sort of thing.
London underground stations - you could come as an Angel of Islington fo example! That works with other lists, like Monopoly places, football teams, Derby winners, film titles (or just films starring one person) etc.
20's cocktail party - very elegant. you have to learn the black bottom!
A place eg Spain which gives you paella and flamenco!
Bazarre - girls as slaves, boys as slave masters (non pc but a lot of fun)
the old favourites - vicars & tarts, school
Another option is to decide what you want to wear, then make up a theme around it!
Have fun & happy birthday