My prediction (which is worth nothing): Prison of Elders (PoE) is going to work like this. You get a wave...kill wave(s) then a boss comes out of one the cells (notice the picture from recent reveal post, looks like a special servitor), you kill it..new wave(s). Repeat. You get "marks" (some kind of currency) with each round with bonuses for performance (I hope). You can then spend the marks on a special vendor in the Reef to buy new weapons and gear. The new weapons and gear require special material to level, which can be won in the PoE.