Homebrewing the Apocalypse! Writing my own PbtA system?

[RedWarlock]

I'm working on a hybrid of Monster of the Week and Urban Shadows for my own setting; there are elements of each that I enjoy, but neither on its own really fits the world I've written. I've read a bit of Dungeon World and Monster Hearts for contrast, but I don't have a ton of PbtA experience overall.

Any useful guidelines for writing my own Apoc-derived system? Pitfalls, common mistakes, etc?

[Powerdork]

Not that I want the answers to these questions specifically, but they're good things to think about.

What game experiences do you want to highlight, and what mechanics are you (/are you not) putting in place to highlight them?
What are the game mechanics that get your players thinking about their characters' relationships, and what are the game mechanics that get your players thinking about their characters' places in the campaign world?
Why are various other game mechanics in your game? What do they bring to the table, and are they worth keeping because of it?
What is the hardest a hard move gets in your vision of the game?