If you add new bits to v4 you invent an incompatible protocol, and you should add a lot of bits so you'll never have to invent another incompatible protocol again. You can also fix the minor annoyances in v4.
Hardware implementations typically do not like variable-size fields. Not just because the total header size becomes unpredictable, but because it means any following fields no longer have a fixed offset, and that complicates parsing.
True future-proofing would require representing address length as an arbitrary-precision nonzero unsigned integer.
Since allowing a zero-length network address format would serve no purpose other than to pointlessly complicate standards definitions, you could trivially and without loss of generality interpret zero to denote some extended-length address length representation to be defined in a future version of the standard.
