I think about there should be some kind of "other" attachment slots. For example if you want to add a flashlight or a PEQ-15, you dont need to replace your foregrip or whatsoever.
The positions of the "other" attachments can always be at 0 0 0 which you can move.

A fifth attachment slot? Would be cool, and if done right could allow things like canted sights or toggleable magnifiers.