I apologize if this or something similar has been suggested already, I haven't seen it so…
Anyways. A strike. That's it.
IF Destiny 2 is based on the cabal, in whatever small update we are going to have, I think it would be cool to introduce a new cabal based strike. On Earth.
So you get a directive from the vanguard to go check out a single cabal ship that has ventured on to earth. When you get there, you see an abnormally large amount of enemies (for a strike) and you know, you do strike things, clear them out, blah blah not important. I would note that this should be a moderately difficult strike… At the end, after you defeat whatever ninja turtle boss they throw at you, there's something to scan…
Ghost sounds frightened/nervous/anxious. Before you arrived the cabal had sent a message: "all clear, begin invasion." Ghost explains to you that the cabal you just fought were only a small scout team of their weaker soldiers. There's an elite force of thousands/millions preparing to annihilate everything. It closes with a line from ghost:
"This was only the beginning. They're coming."