I played some Unreal Tournament 2004 the other day, with a game mode called Onslaught. It is a great PvP mode where players join 2 teams that battle to control the map, by taking over and defending nodes around the map, on foot and with vehicles.
This would not be very hard at all to add to FiveM, using code from my Crackdown Mission mod based on LanceGood’s Arma Mission’s resource.
In UT2004, each team had a home node, and could only take over the nearest nodes linked to each other, so teams were forced to take over nodes until they finally battled over nodes usually 1/2 way between each home node.
This could all be done in FiveM, but to start off just using the objectives/marker code in my resource where the nodes are markers that start off neutral and take 10 seconds to capture, whereby they become the conquering team’s/gang’s marker. These markers could be linked to zones, where the ambient gangs (and ambient civs) spawning there are friendly to the team, but hate the opposing team, until that team takes over the marker. Capturing an opposing teams marker (objective) would take 10 seconds to remove it, and another 10 seconds to claim it.
zones/ambient peds could be left out, and/or when a marker is claimed on the map, armed NPC ped allies for the team could spawn to defend the marker in vehicles and on foot, as well as combat vehicles spawned that the players could use. Hostile ambient peds and spawning NPCs/vehicles easily is all in my resource as well.
Say 6 markers spawn in the map at various locations, the objective would be to claim say 4 or 5 for your team (all active at the same time) for the win. Its like a Turfs Wars variant, except adding PvP as well. A blending of NPCs and players. The map would show the state of the game and what markers/nodes are whose via blip colors, similar to UT2004’s minimap. The markers/nodes would not even need to be linked or required to be conquered in any order in this version, making it more of a free for all.
This would be another excuse to utilize all the combat vehicles in GTA 5 that evolved from the GTAO arms race. 
Here is some footage of Onslaught from probably the greatest Onslaught map I ever played in UT2004, ONS-NMP2-PanaleshSE, a custom map made by someone in the community way back in 2004. Someone finally posted some youtube footage of it back in Feb, so people are still playing it online after all these years. You can see the different nodes in the top right hand side. This would be perfect for large population servers as well, which is what the game mode was designed for.
