I think the year ending #1 might be out of roger's hands simply because of the tournaments Roger might skip. Roger may skip Toronto, Shanghai and maybe Paris (as basel, paris and WTf are back to back whch is INSANE).
If djokovic plays Toronto, Shanghai and Paris, and Roger skips them, he'll have a shot at gaining 3000 points max over Roger.
I don't know why we should assume Djokovic will play everything while Fed skips three Masters. If Federer needs to rest, so will Djokovic. He is not known for playing every week, in fact he has played less tournaments in the past 12 months. Paris is one thing, as it probably cannot be combined with Basel Djoko will play and Fed won't. But that could be just 500 points (If we assume a Basel W for Fed and a Paris W for Djoko). As for the rest, I don't see why Djokovic would play more unless he gets a really bad result somewhere and thus more free time, and that would limit his upside potential.
And, of course, Djokovic is one for five so far in the Masters season, so assuming he will win everything Fed doesn't enter is a stretch.
I hope very much for a sixth year end #1. If Fed wins US Open I believe he will always have at least a shot in the WTF. Even without it he has chances, certainly if it should be won by Murray or someone else besides Djokovic & Nadal.