I like the island model although V really stood out like an actual island to me. San Andreas didn't feel like an island to me, maybe it was the 3 cities or sprawling countryside and varied terrain, I could easily forget it was an island.
Part of me thinks I would enjoy something like RDR but then I often wonder what flying would be like with such a map. I wouldn't be inclined to scrap flying (fun and realism) for the sake of map realism. I think the trade-off of it being on an island is perfectly good video-game logic to fulfill a feature that offers more fun, value and gameplay than say, having the odd existential thought about it being an island. If the map is designed well, most vistas should give you an impression you are on a mainland and not necessarily on an island.
If they lock it off like RDR, they'll have to do an exceptional job of it because what would be immersion breaking is invisible walls, ledges that are normally climbable, non-climbable etc.
The island model might be unrealistic in the sense that the place it's representing isn't really an island. However most locations GTA is based on are by the sea so at least one half of the map is real, it's the just the other side where the land should keep going that's at odds. However, the landlocked model also forces you to suspend your disbelief in that you can't drive or go here or there. At least with the island model, it doesn't restrict you in things you can't do. If you want to over that mountain or building you, you can. If you want to see the other side, you can. That player freedom to me is more valuable that coming to terms that you are on an island.
The thing is, too, islands do exist, but locking off map trickery doesn't exist in real life. What I mean by that is if I see a mountain, I can climb it. If I come across a tricky-rockface that stops me from climbing over it, I can at least get a helicopter and fly over it. I'm never going to come across an invisible wall or some physics defying block to stop me doing so.
Making creative ways like being attacked by bears or coyotes or whatever is nice but after a few times wouldn't it just be so unrealistic that every time you want to leave a place or go beyond a certain point you get killed. The island model doesn't ask that of you. It's just an island and they exist except all they ask you do is believe that it's LA or New York which they do quite convincingly.
When we used to play Road Rash as kids, I would think about how it would be cool to go to places we can see in the map. But the game doesn't let you. You're restricted the road and a few metres either side of the road. When GTA III came about it was like wow. Almost anywhere I see, I can go!
The landlocked map in Skyrim is perfect and it's great in Watch Dogs too. Skyrim isn't a vehicular game, obviously so traversing is done a lot of horseback and on foot. I find a lot of Watch Dogs action focuses on non-vehicular action, although vehicles are main thing used to get around.
I just think GTA's exciting use of Airports, flying, flying missions, versatility of missions including flying should remain so. I don't think it should regress to removal of such big features and having those big features greatly benefits an island map.