I feel as though it's not that unlikely that the packet will have many large additions and significant changes. I know that it's only been a month or so since the last packet, but since the packets are not complete images of everything they have done at a particular point, it's possible that things that have been in the tank for a while will finally show up in the September packet. It's the last chance to get feedback from the playtester pool they've assembled.
On the other hand, it's possible that the large public playtest is concluding because they've determined that they've gotten what they need from it, and this final packet will be just some fleshing out, polish, optics, etc.