The way that they were able to accurately balance the bosses without having to worry about player level in Sekiro was, in my opinion, what made the game so amazing. They knew exactly how far they could push the boss difficulty because they knew how strong the player would be at any given moment of the game.
I doubt it will be. They usually put DLC content somewhere within the world, intertwined with the main plot, so you have to have a character far enough in the game to play it. I believe the entry point is going to be at a late game boss named Mohg.
Makes sense. Might not be exactly my cup of tea, but I do plan on giving it a solid go one day - this will just be more inspiration to do so