Huawei is set to launch the successor to the Mate 10 in a few months, and we know that the official name will be Mate 20, based on this EU trademark filing:
It does not stop there: Huawei trademarked several Mate names—From Mate 30 to Mate 90—on the same day. This gives us a rough idea of Huawei’s naming convention for the Mate series for the next eight years.
Little is known about the Mate 20 as of this writing. Current rumors include a 6.9-inch OLED display and a Kirin 980 processor, with the former possibly the same display that will be used on the Honor Note 10.