iOS 9 will likely have no major design changes. They will probably focus a lot on back end stability to iron out the kinks created with 7&8.
There probably will however be some level of UI refinement on the front end. We could also see the implementation of features that already exist in IOS but are still being restricted to and tested by employees. Control Center toggle customization is an example of something that is being tested already.
Also just because no major changes will be coming to the OS it self does not mean that there is not room for new features... I would not be at all surprised to see Apple focus heavily on updating the system apps in this release. They have been neglected for a long time. And while they did get the lipstick and mascara treatment in IOS... Apple has had to much on its plate to really give them the update that they deserve. They have already started this process with the new music app in 8.4. I expect that we might see other similar app redesigns in iOS 9. The phone app for example, is functionally, way outdated. I would expect updated videos, maps, podcast, iBook, and other apps... A lot of that can be done with out messing with IOS to much.
I kind of think the redesign was a 3 year thing. Year 1 being about looks, year two being about system features and API, year 3 will be about apps/refinement/stability... Then we will have something really solid by the time iOS X comes out.