One of the things that I put some though into is that the PIO block is fast and you can build a custom fast interconnect from that (or maybe even Transputer Lin

One of the things that I put some though into is that the PIO block is fast and you can build a custom fast interconnect from that (or maybe even Transputer Link) and then build a large distributed memory “supercomputer” from that. One of the limiting factors is that RP2040 expects to boot from SPI Flash and there is no documented way for external SSRAM (the Synopsys DW IP that is used by RP2040 as SPI flash controller probably even can do that, but its documentation in RP2040 datasheet seems somewhat redacted).