So, this is just a suggestion, I'm not even sure other players would be that interested in this and I don't know of the technical aspect of this is at all possible. But in many multiplayer games, mostly of the MMO variety, raids are some of the best content to enjoy, tough missions with bosses and a lot to worry about.
But I can imagine something like that being implemented in BFG, where specific multiplayer missions are created where a group of around 6 players (or 12 if that is even possible) coop with large fleets to fight a massive threat.
In the WH40K universe, losing ships is pretty much par for the course, so unlike a lot of the normal gameplay where losing ships is a serious ouchy, having such a large scale engagement wouldn't just be fun, but it would focus on a lot of players working together to finish the mission at all cost. And because there's no campaign map to go back to, losing ship suddenly isn't a priority anymore either.
On top of that, it would be amazing if some kind of system were developed for players to create their own missions, their own challenges for other players to play in, not for rank, or points or anything like that, simply for fun!
That's what I think in any case. But like, I've been trying to deal with the Tyranid hive mothership kinda dealio (probably way too early) and realizing that something like that, on a larger scale, multiplayer and with a lot more ships, could just be a hell of a lot of fun to do!