I am way out in left field on this, but I wanted to put this out there and see what happens.
My prediction is that the Raid Lair is going to be Vex themed.
Now before you throw any tomatoes, hear me out.
The Leviathan is slowly eating Nessus. You can see while standing in the Patrol zone, that parts are being sucked in. Nessus is a centaur that was Terra formed by the Vex. It's crawling with Vex. I believe that the Leviathan sucked in a Vex boss and his minions and it made it's home in the belly of the beast.
I also think that the Almighty is going to be crawling with Vex. Mercury was being slowly dismantled by the Almighty. Mercury was hollowed out by the Vex. I think some of the Vex made it onto the Almighty.