These are a few things I want to see in multiplayer:
There should be a in-depth customization of your character including: physical features, clothing, tattoos, etc.
Deathmatch/TDM should be held in small areas of the city. There should be no pedestrians, and you shouldn't be able to see your opponents location on the mini-map. After every round the map should be chosen randomly or by voting. I would like to see to different types of deathmatch: melee only and firearms/explosives.
I want to a co-op rampage mode: where you have to keep killing people or cause the most chaos.
I do NOT want to see achievements/trophies that are only for multiplayer. (Examples: Auf Wiedersehen Petrovic, Top The Midnight Club, Top Of The Food Chain) These were nearly impossible to get without someone helping you.
You should be rewarded when ranking up by getting emblems, special abilities, or special items. (In GTAIV all you were rewarded with was one outfit for your character every time you rank up, THAT'S IT)
The choosing which games to play from your phone, or having to quit and wait was HORRIBLE. They need to change how you access multiplayer and how you invite people.
This is really all I can think of right now.