I could do with two cities this time for sake of variety and sense of purpose travelling by plane from city to city. Boston and Detroit. Mafia and Irish Mob being the main ones in Boston inspired by The Town, The Departed, Black Mass, Mystic River and Detroit introduces more black gangs in to the mix. I also don't care if geographically in real life Toronto or another city is closer. I can suspend my disbelief if the map is amazing, just like San Andreas. Also cool thing about Detroit is, Rockstar wouldn't have to scale it down a whole lot. It looks about the size of GTA V's Los Santos. So Rockstar could actually do a very realistic portrayal of the city.
Other two-city map I would love is Baltimore (purely inspired by The Wire as Baltimore is like a character in itself in that show. It's a show about that city as much as it is about crime,institutions cops and drug dealers). The other city Washington or Philadelphia. But I might be more curious about Philadelphia.