Here's a Medium post from Matt on the basics of designing for AR-First applications. AR has unique challenges related to:
1) You need to teach the user how to "find" the real-world space.
2) You don't control the space as a designer, like you would in a 3d game or app.
3) The scenes that you place in AR might not fit the real world space. How do you handle that through design?
Check it out here.