Maps from other GTA games is something we have tried in the past but didn't find very beneficial for the server and ceased its development. With the server's playercount some areas in the map are more or less ghost towns and any large map expansions to come will face same fate, or result in lower density of players across the map. Jobs from the transportation industry might benefit from map expansions of this sort, but the general criminal/police playerbase not so much. There are MTA servers that trim down their map to just 1 city so to not keep players too far apart.
Maps of this scale can technically be implemented if you apply some clever approaches to how the world is streamed, but we'd still be looking at a great mandatory download size for something that won't benefit us much while also degrading the gameplay for some of the server population.
We would definitely have to rethink a large part of the server with such large expansions/changes and I'm sorry to say that's not in our current vision