6th gen is more about future emerging technologies than an fighter jet.
Like how you can put all existing 5th gen tech in 4th gen aircraft except stealth.
In 5th gen stealth is already there, now you can design your 6th gen jet to be somewhat more stealth optimised that 5th gen
But stealth is not a defining trait of 6th gen, heck it's not even main trait over 5th gen.
Defining traits that we right now know are.
Mumt( could be put in 4th and 5th gen too(same))
Optionally manned( same)
AI assist( same)
DEW( not confirmed, could be put in "larger" 5th gen and 4th gen with enough power).
Variable cycle engine( same)
Fly by light/optoc( extensive modification but doable, also f35 is already kind of fly by optic, no idea about j20 or su57 if they have fly by optics in lots of areas)
NG high capacity network( designed to process large of information extremely quickly, can be put on both 4th and 5th gen, but will require extensive modification),
Much higher and efficient cooling( hard to put in small to medium fighters like f35)
And we're biulding new airframe for 6th gen, newer and more advance composites can be developed and used.
another thing is 6th gen future aircrafts are all "larger" in size wether it be recently flown chinede j36 and j50( smaller than j36 but still su57 zise), us NGAD, UK tempest.
Though French one seems to be relatively medium size due to carrier compatibility.
Large size means, more range, endurance, more powerful avionics, much better cooling, bigger iwb.
But again f15 eagle compared to f16 also have higher range, higher endurance, more powerful radar and other avionics, much better avionics cooling, but both are considered same gen.