I'm basically on my way to developing a continental scale TvZ campaign involving a cooperative hero game for 4~12 players (plus the potential of 4 to 8 more on top). The story behind it is the rise of Dominion against the UED and the plot adheres strongly to the SC storyline.
I could imagine scaling the campaign to require SCII: Wings of Liberty for the first two levels and SCII: Heart of the Swarm for the third level. On the fourth, I wish for a third installment in SCII, this time highlighting the Protoss and the third installment will be required for playing the fourth and final level of the campaign.
Each campaign level has several primary mission objectives that can be accomplished in 20~30 minutes (typical time to sit down on lunch hour) .
So besides a third installment of SCII, the ability to join a game in progress would be nice.