Would it be possible to add an option for 45° increments in addition to, or a replacement for 90°?

Also for balance purposes, I don't know if this would be possible, but restrict how much an inserter could be turned on its output side in relation to it's input side. Nothing more than 135° in the opposite direction.

Sorry, 45° increments is not something I'm interested in adding at this stage.

I think limiting the inserters to 90° rotations adds a bit more flexibility to the base game without removing too many of the design constraints that make designing factories fun, but 45° rotations go too far. For instance, side inserters still have to be adjacent to the belts/machines they are working on, but 45° inserters don't.

In addition, the game supports 90° rotation quite well, and currently I use the default rotation (R key) to handle changing the insert direction. To add 45° increments to that I'd have to find a way to intercept that rotation and then completely reimplement it myself, which is more effort than I really feel like putting in to a feature I'm not going to use myself.

So this isn't a complete waste of your time, I believe Bob's Adjustable Inserters does what you want.