The "gamebreaker" classes like Librarian or Terminatour could be awarded after reaching a certain score like in Battlefront. Same could go for Dreadnought mode. Player with the highest score gets to become the Dread upon the next respawn. It goes without saying that the higher tiered classes should also be allowed customization.
Obviously needs to include more of the races/factions.
Also, it'd be awesome if they ported Last Stand to Space Marine and allowed you to play as established canon heroes like Commissar Yarrick, or Chief Librarian Tigurius, or Captain Shrike and his awesome assault pack/lightning claws combo.
And the sequel should definitely expand to Battlefield proportions. Being able to play battles between Baneblades, Land Raiders, Avatars, Daemon Princes, Squiggoths, and other epic units would be...well epic.