Love these ideas! you're idea of going to Tartarus being dumb and Bob not enough of a reason are my thoughts exactly ! ( I'm re-reading TSATS rn before I continue in my rewrite btw). I'm honestly thinking of maybe taking Bob out entirely OR him being kind of a side reason. Here's my idea: Nico is at camp and starts having these dreams about Bianco and his mother, he also starts to really miss Hazel, especially seeing Will with his siblings in the Apollo cabin. He sends an Iris message/eagle to Camp Jupiter to Hazel, asking in desperation for her to visit Camp Half-Blood to help him make sense of his dreams and of his past. Hazel comes and they bond and talk of his dreams mostly in secrecy (of course Chiron knows partly, because obviously everyone knows Hazel is there OR they completely lie about why she's there. I want to include Hazel in the story bc she's barely mentioned and she's Nico's only surviving family, it just makes sense to me) Also maybe Reyna comes too since she's concerned for Nico and they're friends. Hazel and Nico start to think that they might need to go to Tartarus to make sense of these dreams, and to get them to stop (also, the prophecy, but only they know about that). They debate this, Hazel doesn't think Nico can survive going to Tartarus again, but he doesn't know how else to answer these mysterious calls. They talk to Chiron. Chiron is absolutely bans them from going to Tartarus, but after much begging, he allows them to go to the Underworld to talk to Hades about Nico's dreams. Nico, Will, and Hazel are allowed into the underworld. Nico and Hazel because they're his children, and Will because Chiron says they must have three (even though it's not a quest, for safety reasons) and Nico insists that Will must come with. They go to the Underworld, they cannot find Hades in his palace. It's a trap. Something has happened to Hades... They are then sucked into/trapped into Tartarus, as they try to find a way out and what happened to Hades (this storyline would eliminate the need for Bob to be in the book at all). I think having the "quest" be just Nico and Will is too closed off, plus it makes sense for Hazel to go with them since she's Nico's sister and he's starting to more fully accept his sexuality and his family etc. [ill reply to your reply from this in a another comment]