DoW3 really should have launched with at least one other game mode, let alone another multiplayer mode. DoW1 had DC's & SS's Planetary Conquer game mode & DoW2 had The Last Stand, both of which would be pretty easy to implement into DoW3 (considering the game already has hero units).
Here's some ideas:
A Dark Crusade/Soulstorm-like campaign which allows players to fight against AI or other players to conquer a cluster of planets (essentially a multiplayer version of DC & SS, taking the best elements from both)
A space-like RTS mini-game where players control a fleet of ships, motherships & spacehulks from their respective chosen race & fight each other in 1v1, 2v2 or 3v3 battles (essentially a smaller version of "Battlefleet Gothic: Armada")
DoW3's 'Last Stand' either lets you build small army's to fight against an onslaught of enemy elite units, or lets you control 3 elites of your choosing, but when an elite dies, they cannot be re-summoned.