[quote=@Marshall]
I vote NPC humans cause otherwise some of us are going to be lax on our character design and development. 
[/quote]
Well people can be lazy with their characters anyway. But on that note, specifically, I love taking on antagonist-typed characters and I do not slack on character development. I'd be willing and I'd have fun with working with the GM and others to make a designated "bad" group for the main group to clash with (with characters just as well developed and diverse).