Loved the idea of Monkey Madness II and Desert Treasure II. They both continue beloved series of quests by the OSRS population while having a unique continuation of the story and raising up the bar of difficulty.
Demon Slayer is one of the first quests many players complete, but I think a demon with 7 hitpoints pales in comparison to even a lesser demon.
My proposal is a master (or comparable) level quest that unlocks a repeatable boss (Delrith, at full power, I hope!) that fulfills the niche for players of medium to high combat (level 85+). I think the upcoming update with Varlamore will fill the need for a lot of medium level players, and Raids already offer much to maxed players, but the combat levels in between are a bit of a grind. The end boss would have a difficulty similar to the quest version of Vorkath - challenging, with unique combat mechanics but nothing too difficult for veteran players.
This could be a good way to tie in the Chasm of Fire, as well. Thematically, it fits and it would perhaps set a more concrete goal for the Disciples of Yama. Surely, the events of Shadow of the Storm was not due to a group that would actually trust someone who called themselves Evil Dave.