According to a recent job listing posted for Incubation Investment Designer, it seems to be the next IP of Bungie will be something about necromancy and dungeons.
Way back in 2018, the game studio filed for a trademark for an upcoming project of theirs titled Matter. Since then, there has been no new information about it, but recently a few job listings for the new IP were posted. The description gave some very interesting details, which could give a clue of what the next mysterious IP could be.
Aside from talking about necromancy and dungeons, another job listing for the Incubation Art Director detailed that the project will be something comedic with lighthearted and whimsical characters. There is also an open position for the Incubation Senior/Lead Combat Designer that has a description, which requires the applicant to work on encounters in which AI will play a part, along with weapon and armor systems.
All of these hints could mean Bungie is really working on a brand new IP or this is just a brand new way of describing the job openings. There is no official confirmation from the game studio, so take these speculations with a grain of salt. It would be interesting though if this was an RPG with a theme of necromancy.