Display:

Likely a 6.1-inch Super Retina XDR OLED Display
120Hz ProMotion Refresh Rate (at least on Pro models)
Always-On Display technology
Processor:

the new A17 Bionic Chip (more powerful than the A16)

Cameras:

Upgrades to the camera system are expected
Potential for a periscope telephoto lens (Pro models)
Enhanced image processing and Night Mode
Design:

Possible shift to USB-C from Lightning connector (due to EU regulations)
Dynamic Island feature likely to be standard across all iPhone 15 models
Vibrant yellow color option highly probable
Storage: 128GB base storage likely

Battery:

Battery life improvements are typical with each iPhone release
Connectivity:

5G is a given, possible Wi-Fi 6E support
Software: iOS 17