Modder group Datehacks have created a Dark Souls 3 mod that allows you to play as the bosses.
Sharing the details on Reddit (via Kotaku), the creators stated that the "Forces of Annihilation" mod permits players to experience the game as "any enemy/boss anywhere [across the] WHOLE map", complete with their HP and weapons.
Datehacks was also behind a similar mod for Dark Souls, but because "no one plays [Dark Souls 1 Prepare to Die Edition] at all" apparently, they hope the Dark Souls 3 mod - which has been in development for three years - will gain more traction.