Tons of Accessories
This is a new iPhone we're talking about here so of course you can count on all sorts of premium accessories from your favorite manufacturers. Look for tons of cases, docks, and more to become available immediately upon release and in the weeks after Apple gets these two devices onto shelves.
We'd also expect at least some iPhone 6 accessories to work with the iPhone 6s. The design changes are supposedly going to be minimal and it's not expected to feature any changes to the Lightning port.