There's been a few mods for Sekiro: Shadows Die Twice, from replacing the giant serpent with Thomas The Tank Engine to more sensible weapon replacement mods - but now you can play through the game as one of over 70 different characters from various franchises thanks to the Ultimate Character Collection mod.
The mod has recently been updated to introduce Arthur Morgan, the protagonist from Red Dead Redemption 2, but also has a whole host of other characters including, but not limited to, Darth Maul and Geralt of Rivia.
Created by modder Eyedeability, the pack is a collection of models and textures gathered from around DeviantArt and other parts of the web. The downside is the mod simply swaps out the player model, and does not replace any voices, but still, it's pretty awesome.
You can download the mod here.