[Update]: The update has now arrived with three new outfits, two new challenges, and plenty of ways to show off your skills.
While Sekiro: Shadows Die Twice sadly never received any sort of DLC or expansion, you might be happy to hear it'll soon be getting a rather chunky new update. Besides a variety of fixes and improvements, the update will add a boss rush mode called the "Gauntlet of Strength" that will allow you to put your skills to the test by facing off against every single boss in the game... in succession!
If that simply isn't hard enough for you, however, there will also be a single-life challenge for you to undertake. How exactly that'll work and what sort of rewards you'll get by completing it, that I'm afraid is currently still shrouded in fog.
The update will also allow you to record a brief chunk of gameplay as a Remnant. You can then share this Remnant with other players and thus help them out with a particularly tough area, kind of like a more interactive version of the classic message system. And if that does indeed end up being the case, I look forward to hundreds of Remnants showing you the best route to chuck yourself straight off the map!
As for the final major feature, the October 28th update will also bring with it a bunch of new outfits. How exactly you'll be able to unlock each of them, that remains to be seen, but more costumes to play fashionsouls with is certainly not going to be unwelcome!
Once the update goes live I'll make sure to let you know, but until then you can keep an eye on any teasers over at the official website.