To me the center of DW is Marines coming from different backgrounds initially and becoming one as a team.
One idea I have in mind is to unlock bonuses for kill-team specialization. Maybe a small bonus on a skill or characteristic, plus some Advances getting unlocked for the kill-team as a whole. My player's kill-team has specialized in missions in the Acheros salients, in particular lesser xeno races that still exist in the area. So some bonus against Chaos perhaps, as well as a bonus to CL(Jerich Reach) when it comes to lesser known xeno realms?
Then we had the idea of swapping chapter trappings, that has been explored in a past thread.
Finally, there is still the question of librarians and if they can learn the spells from other chapters under specific conditions. Or is that secret knowledge that will never be shared? Hard to imagine that it never happens, hard to imagine that happens more than very rarely. Probably only under oath to not pass on the knowledge to one's home chapter.
Any other ideas, guys? One problem in particular that I see is that by the time the team has grown together we have a bunch of high ranking marines that are too good for many missions. They should be heading their own kill-teams instead. Or have served their duty and return home. A lot of GMs have reported having trouble running high-rank campaigns, so that remains one of the major problems, if not the major problem of the game in my eyes.
Alex