I think the term “clean slate” should be used with caution. Avionics, engines, and material science account for majority of th4 design effort of a modern fighter. The actual airframe design is a comparatively smaller portion. It seems entirely possible that a derivative of j-20 or j-31 entirely dedicated to carrier operation is in the works, and externally the derivative may look so drastically different from j-20 or j-31 that they would be called a clean slate design by external observers based on shape and layout. but in fact they share majority of underlying engineering, components, and power plant with j-20 and j-31.
Look at how drastically different Northrop’s intended carrier derivative of the YF-23 is from the land based version.
Actually Richard the Airframe is the major part of any development, look at the J-20, while it may bear a passing resemblance to the Mig 1.44, it is indeed a clean sheet design, and has been a long time in the making, they have been able to fly the AL-31FN as a power plant in the interim, and the J-20 has proven a stellar performer with what is basically the powerplant from the Flanker...
while I am skeptical of the galactic numbers put up for WS-15 thrust, it will be a magnificent improvement and offer increased climb rate and agility by virtue of a much higher thrust to weight ratio, top speed will improve, but not nearly as much as we might hope as it takes an excessive amount of additional thrust to move the speed dial up as drag increases exponentially as the dial moves up incrementally...
in any regard the J-20 was in development a similar amount of time to the F-22, so development is a long lead as we move into 5th gen, and expect even longer to move up to a 6th generation, now we could be ready for 5.5 and we should definitely build a 5.5 fighter aircraft as a stepping stone to 6th gen...
when it comes to the J-20, I don't use clean sheet lightly, the J-20 is a real performance leap from any of the 4.5Gens.... particularly in L/O and electronics....