Not too sure about the specs, but if Nintendo is to continue their partnership with Nvidia, they could use something like the Jetson Orin NX APU which is to be released this year, afaik. This would allow them for a machine that could support up to 16GB Ram with an 8-Core CPU and up to 4TFlop GPU (not that that matters much). The TDP is between 10 to 25w on the stock version, which is in the same ballpark as the original switch.
As for additional specs, I think at least 64GB of expandable storage are required, and they should keep the OLED display at 7-inch, but bump it to 1080p. It should have at the very least 8GB of RAM, with 16GB being ideal.
It's imperative to include backwards compatibility - if it can run previous gen games at full 1080p + 1440p in docked mode it would be perfect.