Speaking of arrows, those and (bolts) are probably the only item I could see being freed from the constraints of the current inventory system.

It would make swapping between different ammunition types a bit more smooth if "quiver" was a separate or even invisible slot into which the currently equipped stack would be placed. If another type of projectiles was equipped, the previously equipped one would bounce back in its place. This way you could swap ammunition with just one hotbar slot and key instead of two, and it would make swapping ammo much less of a hassle especially for gamepad users.