The most likely architecture is Ampere, now that Nintendo is using a 12nm Tegra X1, any new SoC would use a smaller node to achieve higher performance, the Xavier chip is still using a custom cpu and has a tdp of 30 watts.
So the devkit isn't public, there isn't much information on it, but from what I've been able to gather, it would need new instruction sets to be compatible with Switch software. It also has a much larger die area (and thus more expensive) and uses more energy than an A76, while also being a lower performer, though it is on a 12nm process while A76 benefits from 7nm.
Whether the Pro really exists or not, I'm not sure yet, but the configuration only really makes sense as an Ampere GPU with 768 to 1024 ALU with a performance target between 4 and 5 times the current model. It might only have 4 A76 cores and a higher clock, with maybe some A55 cores for the OS and possibly available for games. This is speculation though, I don't have any solid information about a new model coming.
One other thing that we haven't mentioned in this thread is the
delayed "next gen" model that a Nintendo R&D employee seemed to leak to Nikkei a few months back.