Shadowlands Patch 9.1 will bring new Optional Reagents to upgrade your Legendaries and a few new fun recipes.
Upgrading your Legendary
There are two new ranks for Legendary items: Rank 5 (iLevel 249) and Rank 6 (iLevel 262). There is no new recipe for the new rank of base items, instead you can use the new optional reagent Vestige of Origins, which increases the item level of the attached legendary base armor piece by the equivalent of 2 ranks. So, basically, you can add it to a Rank 3 to get a Rank 5, or a Rank 4 to get a rank 6.