GTAIV had some ok game modes like Cops & Robbers and Chopper vs Chopper but it wasn't fully fleshed out. GTAV is for the most part, what the community requested Rockstar do for the next one, since all you could do was play a few co-op missions and play various kinds of Death Match modes. Aside from that, it was just wander around the map. I did have some fun playing with my brother but it was mostly just doing jumps, races or going on a rampage either PvE or PvP. People basically wanted to play GTA but online. Doing missions together, finding hidden packages and all of the side activities. GTAIV didn't offer that. GTAV kind of offers it but they went overboard and made GTAV more restrictive than GTAIV. Both online modes are a let down.
What most people wanted is a Saints Row 2 style of online mode. It's the exact same game online, but with a friend. From what I recall you were never restricted with what you could do in that game, online or offline aside from maybe choosing what mission to do. Whether it breaks the game or not doesn't really matter. I'd be more on board to play shit like Skyrim or Fallout if I was playing the same game but with a bunch of people, especially if it meant you didn't have to play with friendly NPC's whose AI always craps out when it's NPC vs NPC.