However, the only problem I see in a cross-era GTA is a technical problem: it should have two versions of the same map, each of them adapted to the era the story is set in (period-correct buildings, streets, road signs, billboards, etc.), compromising its size and depth in details.
Mafia 2 did it beautifully. It had a huge detailed city, great storyline and the most beautiful settings. I was blown away with the Christmas and Sprint atmosphere of that game. If Take2 could do it, so can R* North.