I'd like to see a whole expansion pack dedicated to encounter cards. I think they could do some interesting things with the scenarios, terrain, and add more plot by spicing up the encounter cards. Written the right way, they wouldn't change of any of the core rules.
A new mission pack would also be very welcome. Funny thing is, I don't feel that the game needs any new space marines. The ones we have now pretty much cover all the bases. I'm afraid that any new space marines would be power creep.
The big question is, does anyone have any idea when a new expansion pack will be released? Has FFG said anything about it?