6. Campaign Co-op
A lot of games with co-op are a welcome but unnecessary addition, either being a separate minicampaign or throwing an extra player in sandbox mode, its fun, for a while, but it would be exciting if an extra player could jump in to your campaign. It doesnt have to wrap in to the games story directly, even if the extra person is explained as a hired gun or guard would be satisfactory. Having another player in those annoying car chase sequences would be a life saver, literally in most cases. Actually thinking about clearing buildings and/or enemy riddled areas tactfully instead of relying on trial and error could be a welcoming addition. Or even just the simple idea sharing the enjoyable, roller coaster ride of a GTA campaign with a friend would be enjoyable - spot that I say friend and not stranger as Im not too keen on having random players failing the mission and skipping cut scenes on purpose, that on the contrary would be enraging.