I think the major innovation this September and for several months beyond that will be functional bands. By that, I mean bands that provide additional functionality and interact directly with the watch either through an improved form of Bluetooth, or the existing data port. These bands could provide additional battery, larger display, additional sensors for items like sugar, levels in the blood and better detection of heart incidents. I expect these bands to cost $75 to 200 each. A couple would be announced in September with additional bands coming out every three or four months. The third party manufacture and sale of bands could also greatly increase.
This would allow Apple to deal with the fact that the Series 3 is such a capable watch and the Series 4 will be even better, that many customers will not want to upgrade their watch for two to three years. However, they might be eager to spend half as much money per unit to buy new, more capable bands every few months or at least once a year.