New hull and CODLAG propulsion for more endurance and speed
The Type-054A/B should have adequate speed for every mission, with the exception of escort for fast aircraft carriers.
But given that China currently only has 3 aircraft carriers, that's only a requirement for 6? Frigates which are faster than the Type-054A/B.
That is not enough to justify an entirely new (and expensive) ship class, not when they already have a mature Type-052DL design which is fast enough, is still in production, and is pretty affordable.
And realistically, it will be at least another 3 years before additional aircraft carriers come into service, so they could continue with Type-054B production for some time.
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Assuming that the Type-057 Frigate is designed to operate with fast aircraft carriers, and that there will be 9 aircraft carriers by 2035-2040, that would be a requirement for 18? Type-057. That does sound like enough ships for a separate class.
Given the pace of Chinese aircraft carrier construction, my guess is that they will build two Type-057, learn all the lessons, and then build a batch.
My guess is that there is also enough production (~20 Frigates every 5 years) that they could sustain 2 Frigates in production:
1. A high-speed Type-057 Frigate to operate with aircraft carriers, with somewhat higher capability.
2. A lower-speed Type-054B Frigate for every other mission. (Along with the older Type-054A Frigates)
I suspect they haven't decided yet, and will wait until they can compare both Frigates in actual usage (and against a notional Type-052D successor as well)