It seems like the Psyker gets a raw deal even though he is one of the leaders and should therefore have a lot more to offer. At present, his weapon choice is extremely limited and skills are too, being very similar.
As Doomandthepain suggested, it might be good to give the Chaplain (and I suggest Psyker too) 6+ skills to choose from. These individuals are the Captains, Sergeants.. they could be treated as such. Also, It's been suggested that the Psyker isn't used in 'end game' highest difficulty setting games.
Give Hellfire to the librarian (it seems sad that a gun that looks so fun to use is restricted to 1 class).
Allow for selection of Campaign Skills.
Maybe: swap Tactical's 'protector' give it a couple seconds more effect and name it 'force barrier'.
Maybe: 'prescience' whereby this skill regenerates others' skills at a faster rate or immediately replenishes or halves skill cooldowns of one player, though hard to know who requires it the most.
Maybe: 'Boltstorm' whereby ALL bolters in use have increased rate of fire for a short time.
Allow 1x Psygate in MP (like a get out of jail free card). ONLY have Psyker enter Psygate when activated, so he leaves a portal (like how it is now) for the others to walk into to use it.
As a DLC and a way of incorporating a different chapter's skins (though many might not want Ultramarines), story wise, how about having Tigurius n company come visit because he's the only librarian in the galaxy that can mind meld with the Tyranids, so teams up with the Deathwing.
Codex is totally needed for all the uninitiated. You've probably noticed vermintides codex addition.
MK2 Bolter sounds really weak like its shooting pips and needs Oomph! Just overlay the normal Bolter sound over the MK2 or something.
'Wait Brother' could be a selection in the quick orders interface. This would also help if you ever choose to have a No Sprinting option.
Change Tutorial by cutting part bashing down walls n collecting relics, so the new player figures it out/ finds it out themselves rather than it all handed on a plate. There was never any need for this in old games.