Revenge of the Marionette achievement section was originally a Bonus Event and now resides in Side Stories. However, this particular World Boss is the same scale as Triple Trouble and Tequatl, and so deserves its own section in General. Origins of Madness is the Living World episode mastery (and should be ideally moved to Story Journal section), so Revenge of the Marionette is the perfect fit to guide players towards this content without it drowning between Return achievements.
Additionally, the meta achievement, 'Master of the Marionette' could reward a title, just like TT and Tequatl do. "Clockwork Vanquisher" is my suggestion for that title. Several new rewards from the event would be welcome as well. Marionette's sword is the perfect fit, additionally, already existing rewards such as Mini Twisted Nightmare, Mini Enraged Twisted Nightmare, Twisted Watchwork Shoulder Scraps, and new variants of this shoulder skin (one for left shoulder and one for both shoulders - could be a Mystic Forge recipe requiring both left and right skins) could be included as well.
Moving the public event to Open World would be amazing as well, but it would require much more work. Some new rewards could boost the longevity for Marionette event so that it gets completed more often, given the majority of the playerbase most probably got all their achievements during the bonus event.