Do they go undercover to hide or to investigate?
I'd rule out Mafia, because that's sorta it's own theme, you know? Works with it's own rules and has certain aspects that would need to be researched. So, if that should be a minor plot theme, it will give you extra (unnecessary) work. And if it's just gonna be mentioned on the sidelines, it's still better to use something that doesn't evoke so many expectations. (I'm not sure if I'm expressing myself very well about this, so...we could talk tomorrow?! *grins*)

Drugs are always good. Keeps it simple, yet conveys you're talking about serious crime and dangerous people.
An evil dictator? *grins* Only if he's going to be important for the story.

I'm so, so curious about this story now. You better do write it! *hugs back*