As I've pointed out before, it will be perfectly possible to achieve the best ending on your first playthrough, even without NG+. I had two core design philosophies behind the inclusion of NG+:
1) It should not be mandatory. If someone wishes to play through the game only once, they should feel fulfilled by that one playthrough.
2) NG+ must add something to the game, not just be a way for players to waste time and play the exact same game with improved stats.
What this means in practice is something you'll just have to wait and see.
I'm not about to go scorched earth and build a total downer of an ending here, by the way. We're currently at the end of act 2, the moment in the story where everything appears the darkest. The upcoming two updates represent a condensed act 3 where, quite possibly, hope may come back.