I agree with the people that said that the core mechanic should really not be changed. I'm a really fan of RPG-ish elements such as equipment and campaign mode, but I do not believe that they fit the game style very well, you can just homebrew campaign rules if you want them.
I think we really need some new events and some new terrain cards. With the terrain cards you'll obviously get new location cards, new missions basically. Keeping in mind that the missions expansion is called Mission Pack 1, we will most certainly get some more.
Same argument goes for Space Marines packs. I do not wish to see all power armour teams, I'm a huge fan of terminators, I play pure terminator armies in Warhammer 40K. I think we'll get a couple more specialised marines like the Techmarine, Deathwing Red Team, but probably not much else in the power armour category.
I don't feel that new teams are really that necessary right now, from a gameplay point of view, but I would like to see some none the less. I play Grey Knights in Warhammer 40K and obviously I would instantly buy a Grey Knights expansion. The problem is if you add chapters like the Grey Knights you need to add their natural enemies as cards too. Adding Daemons and Grey Knights in the same expansion would probably be a pretty big deal, you would also need location, terrain and, possibly, event cards to match the Daemon theme, that's basically another base game release. Still would like to see it happen, but I doubt it.