Memory abundant 64-bit SoC

That kind of SoC is difficult to make? Though self-build capability can attracts many open-source
developers, I think.

How much does “abundant” mean to you? A few MB? A few hundred MB? A few GB?

If the first, it could exist on the SoC as static RAM.

If the others then you need external dynamic RAM chips. The process is quite different to CPU logic. That means a wide and fast bus between them, and a lot of pins on the SoC.

If you find a package with processor and GB of RAM i think it will be multiple dies internally.

I’d like at least 4 GB, prefer 16 GB or more so people can work on the SoC, without setting up a development host environment.

Starting to sound less like an SoC and more akin to a high-end RISC workstation? Not ordinarily a profitable enterprise these days.