Hah, yeah, there are rules. I don't know how strict the judges are, but almost all winning entries across all their events so far have been superhero action outfits. I think only the Fantastic Four winner seems to be the exception with the t-shirt and cargo pants ensemble (and even then, the judges' comments indicate that they viewed the outfits as superhero uniforms).
High fashion popstar designs might go over well with characters that are known to be performers (Dazzler, Lila Cheney, Zatanna, etc). Non-combat garb does well on that site for their non-events (like the civilian-wear section, Off-Duty). If you really want to break from continuity, they also have a section that dresses up characters in a new context.
Well, thanks for the eye opening. I will be reigning it in the future. You doing the Black Terror one?
Ah, the Black Terror one doesn't have rules to it. It's Fan-Art Friday where pretty much anyone does whatever they want. Ostensibly, we're supposed to do Fan-Art of the character's most recent outfit, but over the past year I've seen people just do full-blown, out-of-continuity remakes of the subject.
I'm thinking of doing it, though I'll likely feel compelled to make some changes to the current costume. I like Alex Ross's painting skills, but his work for this character's outfit is pretty ehhh. His design for Black Terror's cowl/mask is … I don't even know what that mess is supposed to be.