We don't know anything about it except that it's an end game 3 player co-operative activity. Besides that, what it will be like is entirely speculative next the way the Developers frame its presentation. Yes, CE was horribly disappointing, and I'm also greatly interested in what this new activity will be like. However, humans are notoriously terrible at making predictions, and this should be no exception. Frankly, we just don't have enough information to make any sort of judgement concerning the Prison of Elders. Perhaps most importantly, we haven't played it yet...