#1. You can edit the way objects stream into view.
#2. I'm pretty sure there's ways to remove the default GTA map also, but there's no point in my opinion because default GTA objects require no extra download.
#3. If you're good enough, you could use EngineRestoreModel and EngineRestoreCOL and EngineLoadDFF and EngineLoadTXD depending on how your server transfers players from city to city, load/restore models depending on which city they're playing in.. You could essentially add unlimited custom cities considering the objects would only change on the clients screen, so the players in other cities wouldn't be effected if they all used the same barrel object for a wall or something.