I would like to see Imperial guard, Necrons, Space Marines and Chaos.
I think that the imperium should be handled like Battlefleet Gothic does: the imperium player has access to all the Imperium's units: Imperial guard, Space Marines, Mechanics etc, however the player can't mix units coming from different races.
I think that imperium at launch can be limited to imperial guard and space marines, the other branches can come later as a DLC.
What races would you like to see at launch?
What about characters?