GTA 5's map is vast, let there be no mistake. It has a metropolis, a desert, wooded areas, small towns, mountain ranges, lakes, beaches, open highways and more. As a multiplayer experience, meeting other players organically is kind of the point of the game. However, it was initially designed with this in mind, and content was balanced accordingly.
In its current state, it is one of the most crowded open world multiplayer games, and one of the least balanced in terms of map-size-to-content ratio. Meeting other players is one thing, but not being able to retreat to a secluded area at all is another. If you have something to do in the city, you have almost no chance of doing it without encountering another player.
In most other MMOs, this wouldn't be an issue, but in GTA Online PvP is the default, and shooting someone as first reaction is the norm. Even if you don't want to piss in the soup of the other player, they might not be similarly inclined, or they might just not know that you harbor no ill will.
Let's be real. GTA Online has its fair share of assholes. This has conditioned decent players to be suspicious as well, and even telegraphing the fact that you're not violent to other players won't prevent trigger happy encounters. Either they will take it as a sign of weakness and kill you for the heck of it, or continue being suspicious because there is a chance that you're just doing it so they let their guard down around you.
The way content figures into this is that certain activities, like delivering crates or stealing cars for their vehicle warehouses, which happen in the regular free roam game world, can lead to players clashing. Having some random less-than-polite bloke rain on your parade isn't something uncommon in MMOs — every World of Warcraft player experienced that one annoying high-level prick who kept hunting you in faction neutral zones while you were still levelling — but this becomes a notable issue in GTA Online due to how frequently it can occur.
Another issue posed is repetition. At the end of the day, whether you're taking supplies to your bunker or to a business, whether you're delivering crates or an imported vehicle, doesn't make any difference from a gameplay point of view. In each case, you need to get the thing from point A to point B without dying, or taking too much damage. The routes can only be so varied when there is so much content crammed into a comparatively small map.
A map expansion would bring with it a number of benefits both for Rockstar and for the player base. For Rockstar, the most notable benefit would, obviously, be money. A map expansion would be big news, thus publicity, and would most likely lead to a large number of players who have left GTA Online behind to return. Additionally, if Rockstar would add a mechanic that, say, charges for plane tickets to reach the new map area, the added costs might drive up Shark Card revenue.
On the other side of things, the players would benefit from a greater amount of space to play in as well as a whole new area to explore — but one issue remains. Simply adding another area won't solve the crowdedness of the existing map. Either content already available would need to be relocated — like no more executive offices in Los Santos, only in the new area, for example — or instead of a map expansion, Rockstar could replace the existing map with a scaled-up version of it. We'd still have Los Santos and Blaine County, and geographically everything would be more or less in the same place, but there would be more land to cover.
In any case, if Rockstar keeps up their current rate of DLC releases, this crowding effect will become much more apparent very soon, and if their plans to keep GTA Online running through 2020 (which seems absolutely viable, based on sales data), they'll definitely need to look at map expansions sometime soon.
