The timing of my suggestion sucks somewhat, but I've had this idea since playing RDR2 when it was still new: a trilogy of RDR2/GTA hybrid games based off the RDR2 map (potentially expanded into new areas in places).
First game set in late 1920s, streets have cars and horse and carts on them, all the fashion, music (jazz), radio shows, drugs, bootleggers, heavy/dangerous industry, advancements in shipping and flight; but modelled onto the exact RDR2 map including every building. Obviously some areas could be unrecognisable other than the road layout if the area had seen a lot of redevelopment since the turn of the century. The game would need to have GTA at it's core and make the 20's feel like the peak of modernity and wit, and not feel like a dingy pastiche of the era. It would need to have echoes of RDR2 through it given it's set not long after the RDR2 story finishes.
Next game set in the 1950s, same map/same ethos, but you take the 20's version of the map as the starting point so there's an evolution of the idea. You could even follow the main characters and businesses on from the 20's game. I think you'd need more map to play with for a 50's game as there are a lot of new industries to explore and find space to place on the map. With that being said, whatever space you think you would need for v2 or v3 of the trilogy, should be added to the 20's edition so you see the entire map expand through the three games rather than having new areas bolted on in later additions that don't have any history to them.
Last game set in mid/late 70's.