Though it seems that it had been considered, the IJN never really managed to put out a good successor to the A6M that adressed its flaws. Assumning a reasonably high degree of success and luck for the navy's carrier aviation early in the war (thereby buying a little extra time to prepare), or a declaration of war later, what direction would the IJN have been likely or able to take with regards to a carrier-based fighter to succeed the A6M? What direction could it have taken?
The A7M (the planned successor), the N1K and an extensively overhauled zero like the A6M8 seem like the most obvious choices.
*Note: Of course, trying to replace the zero would really not have had a deciding effect on the course of the war, let alone its outcome. I am not asking whether the war could be won or lost this way.