As Ragresen said, it's best to pick characters you like. But, with crossplay, dressing as who you like isn't always possible. Though, that doesn't mean abandon all hope. It just means keep in mind what you can and can't do. For each person that's different, but in general nothing with cleavage (to my dismay ;_;).
But anyway, I'll stop rambling and try to make a relavant point. If you do what I do, make a directory of the costumes you'd like to do and look through them as you try to decide which to do. Find points of each that would be good and bad for you, crossplay wise. Weigh how much you like each character as well, and how much you'd enjoy being them for a day or more.
That's not exactly a fool proof method to decide, becauce I still struggle to choose my costumes. But, it can help you to narrow it down, especially if you're more of a decisive person than I am ^^;
But anyway, good luck! And don't be afraid to ask for help or search the web =3