It's not very uncommon in GTA games for parts of the map to change forever when we complete certain missions. Some, like Bomb Da Base II, Trojan Voodoo, Yay Ka-Boom-Boom, Rigged to Blow and Crystal Maze focus around the player destroying certain buildings (and a ship), and they will stay destroyed for the rest of their games.
However, parts of the map with significant changes without the player's involvement are still quite rare. The biggest one I can remember is Callaham Bridge, that is under construction in LCS, and is completed as we progress through the game, and is under repairs in III, and is completed when we progress through the game. Why can't more parts of the map have the same special treatment?
The real world around us is always changing, but some parts of GTA worlds just feel kinda dead, as in they don't make progress. Take construction sites as an example, where you can clearly see workers doing their job, but the place stays the same from begining to end. Would be cool if, with each mission or sets of missions, said construction site's model was updated to a more complete state until eventually it becomes a complete house/building. The same can apply for lots of other stuff: Abandoned buildings being demolished, road construction appearing and disappearing, props changing places, buildings changing colors, 'For Lease' buildings being bought and becoming stores (and vice-versa), ships arriving and leaving ports, etc.
A pro to this would be more places to explore, as you'd get a different experience with each mission passed, and the feel of an alive world, where everything is subject to change,just like our own world. It would also make players more excited to complete missions to see the map progress and explore. As for cons, we'd have models taking space in the files only to be used for a short while, and it would take significantly longer to program and test for glitches.