I am thinking of the big strides Patek have made into SS complications over the past 15 years:
-Moonphase in the Nautilus for many years
-Chronograph in both Nautilus & now Aquanaut
-Dual time in both Nautilus & Aquanaut for a few years now
-Annual calendar in both Nautilus & 5960A
Perpetual calendar was a potential choice and we saw Patek choose G metal, so that one is closed for the time being.
Rattrapante is a grand complication so I know it can never be done in SS.
Then, my next logical thought is WT function and if the 5131P represents the modern king of WT, then perhaps the cycle can start again with SS in a few years once 5131P production is stopped. Obviously no enamel dial anymore. Something in the mould of the 5930G in SS or Ti.