Material
Carbon fiber sheets are most common - light and durable
Conducts electricity so everything needs to be well insulated
Blocks signals so need to place antenna in a good location
HDPE also an option but not as strong
Arms
Easier to fix aircraft (and cheaper) if the arms are replaceable
Having unibody (arms not removable) could be less machining
Shape
6 Arms
https://www.diva-portal.org/smash/get/diva2:1216528/FULLTEXT02.pdf
4 Arms
Dead Cat
allows space for a camera at the front (and no rotors in the way)
True X
Same stability on every access since the motors are all at the same distance
More aerodynamic
H Shape
Wider so it could fit the drone more easily
Sites:
https://dronenodes.com/drone-frame-racing-freestyle/
Stages
2 Stage:
Can carry more stuff
Could put a nice 3D print insert to enclose everything
Another good resource: https://blog.dronetrest.com/fpv-quadcopter-frame-buying-guide/