Expect the new iPhone and very likely an Apple Watch

New iPhones are the only absolutely guaranteed product to come in September

Origen: What to expect at Apple’s September event –and what not to

 

New “iPhone Pro,” or “iPhone 11”

Last updated: September 2018
Likelihood at September event: certain

Apple Watch
Last updated: September 2018
Likelihood at September event: certain, to some extent

MacBook Pro

Last updated: The 15-inch MacBook Pro was updated in May. The 16-inch model (obviously) has yet to be revealed.
Likelihood at September event: low

New iMac and iMac Pro

Last updated: iMac 4K and iMac 5K in March 2019. The iMac Pro shipped in December 2017.
Likelihood at September event: low

New iPad Pro

Last updated: iPad Pro was updated last October
Likelihood at September event: 50/50

New iPads

Last updated: iPad Air and iPad mini were updated in March 2019, but the regular iPad was March 2018
Likelihood at September event: 50/50

Other hardware

Apple does also make AirPods, which have been updated once already this year, and the Apple TV 4K, which hasn’t.
Likelihood at September event: low