Vex. Especially if you read the grimoire on them. I think they will play a much large part in the next game. It talks a lot about how they can run simulations and how the people might be in one In the grimoire.'in the opening cinematic of the game you see the 3 cosmonauts walking away from the spaceship an as they walk the scenery is constantly changing. Are we in the game being simulated? Also the stranger talks about how she comes time and time again and sees the same people dead...sometimes herself. She also says when we first meet her on Venus they(vex) brought us here. Maybe that doesn't mean physically, but brought us into the simulation. Anyway it's just a theory and I especially love the vault lore and kabr.