Bullet sponges. Also everyone is saying that the emperor of the Cabal is just gonna come raid us personally because of some signal. That's not gonna happen. This empire has survived for a very long time and it would be a major tactical disadvantage if he/she were to travel away from the base of operations
Edit 1: I'm not saying a Cabal Raid wouldn't be awesome, despite what I have said. The only thing is that it must be done in a different fashion than the other raids.
Like one guy mentioned, perhaps they could give it less of a sense that you have to kill everything in sight but a sense of being completely outgunned yet you must still must prioritize and make it to the boss. Also I was thinking maybe they could do what EVERYONE wants, and that's usable SHIPS (not just as loading screens). Perhaps at one stage we have one set of the team on foot while the rest are preventing Cabal reinforcements in the air