There are a couple of things to consider about September 2016 and whether or not it's slow. Halloween Time usually begins around mid-September. This year it began on 9/11. Next year I'm not sure if Disney would actually start it as early as 9/9 or if they would begin the season more around 9/12 or 9/13, but Halloween Time is very popular and brings in a lot of guests. This year the parks have apparently been very crowded in September -- last week there was even a brief time when
Disneyland closed to capacity, which never happens in September. I think that the crowds this year are partially due to the Diamond Anniversary, as well as Halloween Time, various other events and the fact that a lot of things are closing permanently or temporarily, causing many people to go to the parks to see them one last time.
... Which brings me to my next point. Supposedly, the Diamond Anniversary festivities are scheduled to continue through September 5, 2016. I don't think we know what that means, exactly, in terms of the entertainment schedule and decorations. Disney's summer season will technically end in late August, but I guess that they want to at least run the Diamond Anniversary shows through Labor Day of 2016 (probably beyond that, I would guess). After 9/5, I think they will begin the transition into Halloween Time mode and maybe remove the Diamond Anniversary decorations for good. Up until 9/5 I would expect there to be a lot of people in the parks.
There will likely be some sort of hotel discount covering some portion of September and ending in late September, but it wouldn't come out until the summer.