"Instead of taking the best of the dolmens (predictable rotation), the best of the geysers (scalability based on number of players), and the best of the dragons (map location and health indicators) and adding them together to make a fun and dynamic world event scenario, they gave us....... harrowstorms." https://forums.elderscrollsonline.com/en/discussion/comment/6850523/#Comment_6850523
There is incremental patch maintenance the 16th, but we won't know what's getting changed until they publish the patch notes at the end of maintenance. It shouldn't be anything major.
Update 25 and its natch potes should be available (on the PTS) on January 20, if they follow the same schedule as last year. But that should be the Q1 dungeon DLC pack due in February/March, not the Skyrim chapter anticipated for Q2.
The initial patch notes for Elsweyr came out (for the PTS) on April 15 of last year, so we can probably expect a similar date this year for Skyrim's.
We have no firm dates yet, but hopefully this gives you some idea of when to expect what.