Would there be any way to fit in a "Crime Lord" into League's lore?
What do I mean by that? Well, simply put, we could refer to Rupert Thorne (Batman franchise), The Kingpin (Marvel comics), Prince Xizor (Star Wars), etc. Any of those could present a good example of a Crime Lord, if in a somewhat stereotypical fashion.
Gangplank might SOMEWHAT fulfill this, but he's more "the Pirate Lord", which differs in that he's far less clandestine. IMO, he doesn't fit just right.
Additionally, I do not think that adding in a Crime Lord as a Champion would be the most ideal way to do things, but perhaps making one as a lore-only character would work out. His presence could be felt all across Valoran's cities, but he would not try to take on any Champion in combat; he'd just let his lackeys do the dirty work.
Could this work?