iOS 12.2
It may or may not get any stage time at the event, but it’s almost certain that we’ll see the release of iOS 12.2, most likely on the very same day.
Apple has already been ramping up its iOS 12.2 beta cycle, with very minor “polishing” releases now only coming a week apart, and iOS 12.2 will definitely be required to access Apple’s new premium news service, which by itself will be reason enough for it to be released concurrently with the event. Further, iOS 12.2 also contains code for new Apple Pay and Wallet features, and the necessary code to support new AirPods and Apple’s AirPower charging mat, which are also likely right around the corner.