Something that quite a few people missed which was added in Patch 10.2.5 was the addition of the Dreaming Equipment Chest item to Lindormi, the Mythic Keystones NPC in Valdrakken.
The Dreaming Equipment Chest costs 350 Flightstones and is account bound, meaning that it can be sent to your other characters. However, it requires the character you are buying them on to have a Mythic+ rating of 1000 for the item to be purchasable.
The tooltip of the item suggests that it will not be available once Season 3 ends on April 23rd. However, unless you really want to gear up another character to play immediately in Season 3, remember that this is likely to not be the best use of Flightstones if you can use them again in Season 4 on upgrading those items of gear instead.
When opened, the Dreaming Equipment Chest rewards you with a random item level 441 gear item on the Veteran Track from the Mythic+ Dungeon loot tables, meaning each item can be upgraded up to item level 463; using Whelplings Dreaming Crests, Drake’s Dreaming Crests and Flightstones. You can buy these chests on a character that has enough Flightstones and then send the unopened chest to the alt you want to open the chest to receive the gear on.
The items contained in the chests can be used in the Revival Catalyst, which means an instant 4 set for your character if you have enough Flightstones to buy them and get lucky with which items you get (whether its a Head, Shoulders, Chest, Gloves or Legs slot item).
Lindormi: Dreaming Equipment Chest Vendor Location
Lindormi is located just outside of the Great Vault, next to the large staircase leading to The Seat of the Aspects tower.
Hopefully this new gear helps you get a new character gearing quickly and you can get more prepared for Season 4 later in the month!