If we're going to be Loyalist Marines, then I say the next campaign should be a multi-pronged crusade with 3-4 Space Maines chapters involved, each with their own forms of fighting. So Raven Guard would use Jump Packs, Black Templars would be close range/on foot melee, Dark Angels/Imperial Fist would be more ranged based, etc. This way you capture the scale of war in the 41st millennium and appease the fanboys by having lots of different chapters to play as, not just the smurfs.
As for MP: everyone says the MP is broken, I just say it needs adjusting. Massed spam of Assaults, can be annoying, but a Tactical class can roll out of the way of any AeO ground pounds and unload some dakka in their face. If you're trying to play MP like SP, then you're probably going to get owned. You do get the ability to copy your killer's class, so if you think the jump pack + thunderhammer is cheap, grab it, see for yourself and go on a tear. You can do this from level 1, so use it.
MP improvements:
MORE GAMEMODES! I want to blow stuff up, so give me a Rush/Demolition-type mode. A free-for-all would be kinda good too. The Dreadnought idea seems like it could be fun and makes me think of the Monster-mode from UT.
Make getting into a lobby way faster, it takes far too long right now.
Minor weapon balancing. The bolter needs to be a little better. You can kill someone in a single mag, but it takes that whole mag. All it needs is a little buff.
The between game lobbies are 60 long, but the customizer takes 15 seconds to load. This makes it hard to change perks between games as you don't have a lot of time. Either makes the customizer load faster, or have the lobbies be longer. I prefer the former option.