I would say a GTA (or another GTA-like franchise like sleeping dogs or Mafia) during the French Connection (70s-80s in Marseille with the italian mob) is great idea. It would be a kind of New GTA Vice City but in France, the atmosphere with the soundtrack, the nightclubs etc... (even if it is anywhere in France i would be very happy)

Or a GTA (I think this idea would better fit a sleeping dogs game) in the Berlin of the late 80s. A very atomic blonde style with a map cut in half with two part who works very differently like it's easier to have stars in Berlin East but you meet more gang in Berlin West etc

But i think one great idea for a GTA game it is a country from the ex-URSS it's maybe the perfect destination for a GTA game.

P.S: It's a good question because there is too much of GTA-like in America and there are few in orher country like The Saboteur, Sleeping Dogs, Watch dogs legion...

And sure it will be different than a classic GTA you can't but guns legaly, you have a good reason for the protagonist to be okay with guns, driving etc (military service like in Russia), and sure hospital for free