As early as 1940, the navy considered a replacement for the Zero and this was 16-Shi specification. Almost immediately it ran into difficulty as falling on Mitsubishi, whose dsign staff were overworked at the time, headway was not made and was withdrawn, but re-instated by the navy in April 1942. To this renewed spec, 17-Shi, the A7M was built. Again, the reason behind the delay in building the first Reppu was shifting wartime priorities and demands made on Mitsubishi by naval staff, for other work. Sounds awfully familiar. Also, in March 1945, Mitsubishi's Nagoya factory was bombed out and the A7M mock-up and thousands of drawings were lost.
Variants of the Zero were progressively upgraded, as we know, and the naval staff responded belatedly to Mitsubishi's repeated requests to improve the Zero's performance with new engines and modifications and it was the disappointing performance of the A6M5c and '6c that forced the navy to do as Mitsubishi rcommended and fit the Kinsei engine, which Mitsubishi had preferred for the Zero from the outset, but the navy insisted on the Sakai. This new became the A6M8, which first appeared in April 1945 and the Kinsei was a 1,560hp engine, modifications such as improved fire extinguishing systems and self-sealing tanks were added and the nose machine guns removed. It was hoped that this aircraft could successfully take on the Hellcat, but they never met in combat as no A6M8 was completed before the end of the war owing to the chaotic state of the manufacturing industry in the last year of the war.
In hindsight, the biggest hindrance to plans to replace the Zero appeared to be the Imperial Japanese Navy staff. By the time they acquiesced to the manufacturer's requests, it was too late.