Had this thread before.
1. Really good story with no multiple endings(hated multiple endings).
2. No spoilers on trailer or game play like today's games
3. New Multiplayer modes that keep you going, like the examples from above linked.
4. New Ships, factions and Research tabs. Example from above for research(linked)
5. Not a MMO. For some reason that Relic/THQ decides it should be an MMO(God No!)
6. New possibilities. Examples: Bring your passive upgrade ships from single player into multiplayer / set those passive upgrades in multiplayer option before starting the game. (Skill Tree Upgrade idea is passive, upgrade once, limited/restricted, upgrade one thing you cannot upgrade the other(link above)) or playing a different faction for another campaign.
7. Don't change the game play, back to HW2 style with a more upgraded format at the most, learning curve isn't too difficult or it is not like Sin of the Solar Empire.
8. Probably a special Bentusi Campaign or some other special DLC/Bonus Campaign.