At the time of writing no new changes or features have been discovered, but we'll update you with any large ones introduced in 12.1. This is the first of many betas for the iOS 12.1 update, that should be out to the public in November.
Apple also released the first macOS 10.14.1 beta, tvOS 12.1 and watchOS 5.1 betas to developers as well. The iOS 12.1 beta does not include restore images for the new iPhone XS and iPhone XS Max.
Release notes have not been released yet, and the update isn't out for public beta testers. More soon.
Update 3 (26/9): You can change the aperture of images before taking a photo, it seems. A new f/ button is at the top of the camera app. (iPhone XS/Max)
Update 2: Apple has released beta 1 to Public Beta Testers — with a Group FaceTime as well.
Update 1: Group FaceTime is back with this beta. There are no other changes or features found in Beta 1 but expect more in the second beta.