Speaking of Never-built designs, I've just read up about this on Wikipedia about the Akagi
The ship's anti-aircraft guns were grouped amidships and placed relatively low on the hull. Thus, the guns could not be brought to bear directly forward or aft. Also, the island blocked the forward arcs of the port battery. As a result, the ship was vulnerable to attack by dive bombers. The ship's 12 cm 10th Year Type guns were scheduled to be replaced by more modern 12.7 cm (5.0 in) Type 89 mounts in 1942. The anti-aircraft sponsons were to be raised one deck to allow them some measure of cross-deck fire as was done during Kaga 's modernization. However, the ship was lost in combat before the upgrade could take place.
Kinda makes me wonder what other modifications would've taken place.