Mafia 3: Definitive Edition didn't sport any new additions like Mafia 2, but YouTuber Sliderv2 has been rooting around in the files and found a couple of interesting things, most notably a large but unfinished map of Berlin.
Berlin is quite far away from Mafia 3's fictional city of New Bordeaux, but it's likely the map has nothing to do with the series at all. Developer Hangar 13 was working on a Cold War spy game, Rhapsody, which followed a Jewish protagonist as they snuck around Berlin and tried to avenge the murder of their parents. It's highly likely, then, that the map was designed for Rhapsody.
According to a Kotaku report, Rhapsody underwent some massive changes during development, ditching the spy theme and the Berlin setting. There was talk of superheroes and using music as a weapon. At one point co-op was being considered. Confusion about what the game was about and subsequent layoffs meant we never got to see either version.
Hangar 13 apparently didn't start working on Rhapsody until after Mafia 3, suggesting that the map was retroactively added and deliberate. Perhaps Hangar 13 just wanted to get something from the game it was working on out there in the world.
Though quite large, there's not much to the map. You can see some landmarks, like the Berlin TV Tower, but the buildings are just empty grey boxes sitting on top of a satellite map. It's still pretty recognisable, though, and all the grey is at least in keeping with East Berlin.
Mafia 2 and 3: Definitive Editions are out now, but you'll need to wait until August to play the full remake of the original Mafia.