No expansions have been announced yet for Sekiro: Shadows Die Twice, but FromSoft's previous games, Dark Souls and Bloodborne, have received important story expansions. They tend to tie up a lot of dangling story points (while delivering some exceptional boss fights and beautiful new areas), and Sekiro certainly has plenty of mysteries to explore. Let's take a guess at some new places a Sekiro expansion could go. Fingers crossed there's one in development!
Warning: the following contains a ton of spoilers for all of Sekiro: Shadows Die Twice.
To the past, to visit Tomoe
Hat tip to the Sekiro wiki for the video.
You hear about Tomoe quite a lot throughout the game, in ability descriptions and booze chats, and in Genichiro’s second form when he uses Tomoe’s lightning techniques. She was a powerful warrior, almost able to outclass Isshin, and she either found a way to reach the Fountainhead Palace or originated from there. She appears to be dead during the events of Sekiro: Shadows Die Twice, but as the Hirata Estate area shows, From games can easily travel through time.
Dark Souls and Bloodborne's expansions both served as prequels. There's clearly a lot of history between the Owl, Lady Butterfly, Isshin and Tomoe that a Sekiro expansion could really dig into, but Tomoe is the most mysterious of the bunch. Was she a human who managed to reach the Fountainhead Palace, or one of the Okami? Why did she teach Genichiro her arts, and what was her clan like? What was her relationship to Takeru and why did he take the branch to the mortal realm? An expansion could tie a few of these questions up, and Tomoe would probably make for a great boss fight.
To the era of Isshin's prime
If you side with the Owl when he asks you to betray the young lord, you learn that Isshin and Emma are obsessed with killing Shura. Shura are individuals who have fallen into a killing frenzy and are on a path to demonhood. If you reach one of the longer endings you discover that the Sculptor's visions of rising flames were a prophecy forseeing his transformation into a fiery demon of hatred. In the Shura ending Sekiro's left arm catches alight and he goes on to kill thousands throughout Japan.
This is interesting because Isshin makes several references to having faced Shura before. He's also credited with building the Ashina clan up and squashing rebellions during his prime. Who were the shura that Isshin hunted in this era? In the main game you can wheedle a few secrets out of him if you give him enough booze, but I'd take an expansion that explores his past in more detail.
To the west
In one of Sekiro's endings the Wolf leaves Japan with the Divine Child and makes for the west to "the birthplace of the Divine Dragon". That's a great setup for a sequel, but an expansion could cover it too. The setup has obvious parallels to Journey to the West. Could we see Sekiro travel to China?
'The west' is rarely mentioned in the game, except for one curious description explaining that the Divine Dragon came from the west and put down roots in Japan (perhaps because of the area's rich earth, which supposedly attracts gods). Before you fight the dragon's full form you chop through dozes of corrupted, vomiting dragons of the old tree. Is their dragon's sickness caused by the infestation you fight throughout Sekiro, or did the dragon bring it from the west? Why did the dragon travel to Japan in the first place? Why is it, too, missing its left arm?
An expansion could also do more to explain how the Divine Heirs work, and reveal how immortality, the Palace waters, carps, and Fountainhead nobles all fit together. Failing that, an expansion could always go deeper into the Divine Realm, which would mean more gorgeous palaces and cool gods to fight.