I think that Prison of Elders is clearly a series of Bosses. After all it's a Prison. It's where the Archob Priest came from. The picture looks like a bigger servitor. Probably a mix of repeat bosses, and also hopefully some new bosses.
It really looks like Bungie planned to include a six player raid and then their plans changed. I think this because of all the massive amount of datamined info that was pointing towards a new six player raid. Probably HoW was released in May instead of March because of these plans changing. I expect PoE to feel rushed.
Why did their plans change? IDK. Maybe the planned raid sucked. Maybe they internally saw CE raid and all its cheese as a huge flop. Maybe they are trying to reach a larger audience with a less hardcore PvE activity. Maybe they wanted the ultimate endgame PvE activity to have MM and it was hard to do that successfully and have it be fun with their current six player designs. Maybe they needed more content for Comet in the fall. Maybe they'll try to sell is an additional raid over the summer for more money.
I believe that's the case. To create something similiar to what was VoG is a hard feat. I also think that a good time for them to release a raid would probably be between HoW and Comet. So they can continue working on that raid, make it a good one, and keep us playing during the summer when it's released.