Patch 8.3 will introduce the Visions of N'Zoth. This likely will be the last patch for the Battle for Azeroth and it has been announced that it will be released January 14, 2020.

8.3 is currently up on the PTR and, while changes are still occurring, here are some things to be aware of:

• Titan Residuum will be reset.
  
  Update: At the start of patch 8.3 (not season 4!) any Titan Residuum that you currently have will be converted to silver at the rate of 1 TR = 1 silver. So it's best to spend any TR that you have on gear that you can use prior to January 14th. After season four starts on January 21st, season three gear will scrap for zero TR. They have also announced that you will not be able to acquire any TR between January 14th and January 21st, so there's no point in buying Azerite gear prior to January 14th with the idea of scrapping it later.
  
  Starting in season 4, the Azerite gear vendor will sell generic 445 gear for 175 TR and generic 460 and 475 gear for 900 and 4,750 TR respectively.
  
  
• The Heart of Azeroth which was capped at level 70 in patch 8.2.5 can be upgraded to level 80 in patch 8.3. A new minor slot opens up at level 75. If your Heart of Azeroth is capped at level 70, consider stacking your rep for each faction up to 19,999 and pick up your chests when 8.3 goes live. Since each chest awards you 2500 AP (which would otherwise go to waste), this can be pretty helpful. It takes 17,500 AP (or 7 chests) to go from level 70 to 71. Note that the AP is not contained within the chest but, rather, it is awarded when you pick up the chest. So stacking the chests prior to the 8.3 release won't work. There is an addon called Paragon Reputation that can help you track your rep beyond 10,000 and up to 19,999.
  
  
• You should also try running a mythic+ dungeon during the week of January 7th which is the week prior to the patch going live. If you complete even a mythic +7, your weekly mythic chest will contain something like 3,500 AP.
  
  
• Once season 4 goes live, mythic +15 dungeons can drop 465 gear and award 475 gear from your weekly chest.
  
  
• There will be a new raid released January 21st. The raid is called Ny'Alotha, the Waking City. Currently the gear drops are shown to be 430-440 for LFR, 445-455 for Normal, 460-470 for Heroic and 475-485 for Mythic.

See the Wowhead article for more information about patch 8.3.

See the Wowhead article about what to do on patch day.