Brings to mind Need for Speed Most Wanted's Fairhaven, which has a bastardized version of Boston in it's downtown core with a lot of the local landmarks (Rowes Wharf, City Hall, the Custom House, etc.). Not nearly geographically correct but it's fun to spin donuts behind the Moakley Courthouse.
Meanwhile, here's the absolute worst rendition of Boston in a video game - arcade style IndyCar racer CART Fury. The Hancock appears twice - once to the north of the Garden and then again in Charlestown, apparently. The solution to our city was not to bury the Central Artery, but rather was to bank it like Daytona.
Based on how popular San Andreas and later GTA V was with the "region" concept of Los Santos I think these GTA maps are probably too small for what we will see in (a hopefully eventual) GTA6. If Rockstar were going to do a GTA6 in the New England area I image it would actually be something like one corner of the map is "mini-NYC" (Liberty City) and then New England as a mini-Boston/Providence/Worcester/Hartford type setup.
These maps would have worked in the GTA III/Vice City-era of the game.
Will be interesting to see what they finally opt to do, they have made so much money on GTA:V Online
^ I love your idea of a Northeastern region. It'll never happen but it's fun to imagine: NYC and Boston with a Cape Cod/Long Island hybrid in between, then up north you could have the rural/mountainous area where they make a nod to the current polarized climate where they have Vermonters aka LibErALs and New Hampshirites aka CONsErVaTivEs taking potshots at each other. Dumb them both down until they're practically Looney Tunes characters and then you can poke fun at both sides while also making light of it all. Not trying to get political... just trying to think of how Rockstar could realistically tackle the current polarized climate.
I'm going to say this: I love driving in Boston.
At off peak hours (10PM to 6AM), it's like an F1 level video game if you drive a little faster than you ought to. Going Eastbound on I-90 after the Allston gantry into the Pru tunnel? The financial district on a weekend? The Bowker Overpass dip and rise set entering Storrow Drive EB, followed by The Chicane starting at the Hatch Shell weaving to Leverett Circle? The 2000 feet approaching the Zakim SB on 93? All approaches to downtown from the south are GTA worthy views. If all they did was mock up the city from Google Earth and procedurally or algorithmically remove the cars, I'd dutifully drop my $40 on the DLC.
At all other times, you are going slow enough to actually look at how much great stuff is going on, and what an awesome town this is.
The single best bit of road within 128 (barely) is the onramp to 2 EB from Waltham St. in Lexington. It's a decreasing radius curve that tightens up big time with little warning, and you suddenly really have to crank the wheel to keep on the ramp. Sort of a personal-sized version of Fuji Speedway's final right-hander complex.