Timeframe: US Prohibition era (as in early 1920ies to 1933 or so)
Location: New York City
Plot locations: Speakeasy, streets, hotels, ballrooms, shady, secret gambling rooms, other places;
Possible Character types: [all characters will be human] Bartenders, dancers, entertainers, prostitutes, corrupt politicians, gangsters, mobsters, cops, pimps, business owners, reporters, artists, private detectives;
Possible themes: Crime syndicates, Mafia, blackmail, action, drama, illegal activities (gambling, prostitution, alcohol, etc.), social class differences, romance (as long as it doesn't take over the plot), violence, murder, conflicts between characters, love/hate, passion, forbidden relationships/friendships/connections, racism, homophobia, tragedy, .... and probably a ton more.
Inspirations: Boardwalk Empire, Good Fellas (just the overall theme, not the timeframe), The Untouchables, Scarface, Once upon a time in America, The Great Gatsby, Miller's Crossing, Bonnie and Clyde (of course!), Last Man Standing, Bullets over Broadway, Road to Perdition, Casablanca (again, the setting, not the timeframe);