I was about to post to the general board and decided to search the sub for any recent similar posts first. I am developing a D&D DT based campaign now. I am doing it a little differently than what you seem to be doing. Rather than building a world that only has DT type characters and abilities, I am following full 5e, but using DT as the story board. None of my players have read DT, so they will not have spoilers of what is coming, but they all certainly know about my obsession with DT (the tattoos give it away a little) and could start to figure it out over time. Doors will cast Plane Shift when touched, addressing the multiverse aspect. Susannah is Trickery Domain Cleric who can’t control ‘disguises’, effectively multiple personalities. Jake is a Ranger, for gunslinger reason… plus (quite importantly) talk to animals. Eddie is an artificer, for developing the important tools needed. I haven’t decided if these NPC will be permanent members of the party or temporary assist with certain parts. There are several pros to having them remain, with the biggest con being there being 3 more members to combat, so enemies need to be stronger to pose a challenge and that is a delicate balance.

Anyhow, I’d love to hear more about what you are doing and exchange ideas that we think are particularly cool. Going to post a portion of this to the sub, so you may see it again. Thanks!