For me there's lot so i created a list on what would be sick for mp on GTA 5

Free-roam - Its the most fun to have in online since you and 15 (or 31) People can battle it out in the city as your own play ground which makes even buying the game just for online worth the price. So if gta 5 had it i would like it if they included more stuff to free-roam like bikes, more places to explore, places to hid out, secret weapons, being able to do some role play in the city.

Deathmatch - Dm or tdm never was my favorite mode on gta 4 since i didn't really play much of it but if GTA 5 included this with new features like smaller parts of the map and a bit more even it would be great.

Bank robbing - This would be so cool if they could included this into online. Imagine you and a bunch of buddy's driving down to a bank in a van then opening the front doors to the bank while worrying about how you will escape the cops would be sick! In gta 4 you could sort of pull it off but it wasn't meant for it but if R* got ideas from games that have done that before (Kane and lynch) that would be fantastic. And maybe even including a feature on where you can map out on the map where you want the robbery and where the cops will spawn and you could upload that in game so others can download that and play that feature that you made.

Gang Wars - So it be 5 gangs in game in where players would spit up into those gangs and they have a chose on where they would want there hid out. After that they can go around and attack other gangs, Become partners with other gangs, attack gang base's and even sell drugs and guns to npc's and people in-game. That would be awesome.

Now i can go on and on about what should be in online but i don't have much time to put all of that down so what do you guys want in online?