I would say you could do a Homestuck group but then realised you didn't want to do such a popular costume :P
You could always do a Bleach group in school uniforms or even their soul reaper outfits. Bleach is an older anime so you don't see too much of it anymore.
If you want to try crossplaying you can always do a Free! group, or solo. I'd suggest not in their swimsuits if you're going to crossplay for obvious reasons, but they have school uniform variants as well as casual clothes, track clothes and even a 'desert' version (see here what I mean)
Also, if you want to do something that almost no one will have but won't necessarily be recognized, you could do an Ava's Demon group, Ava's Demon being a really fab webcomic you can read here