Developer betas of iOS 12.1.3, watchOS 5.1.3, and tvOS 12.1.2 are now available. These are version 3 of the betas. The release notes mention bug fixes, and nothing major is known yet.
Hopefully iOS 12.1.3 will fix the cellular bug affecting users of iOS 12.1.2 (including myself). Apple did release an updated version of 12.1.2 to correct it, but it was only available if you updated your iPhone via iTunes, and not as an over-the-air update.
Some of the features that came with iOS 12.1 included Group FaceTime and eSIM support, as well as 70 new emoji. Incremental updates—iOS 12.X.X—often bring bug fixes, security updates, and other minor features. The next big version of iOS 12 will be iOS 12.3, which will probably be released this spring.