It is amazing how few options are left if you want to build/design your own 16-128 node multiprocessor retro-supercomputer. In terms of what is actually still in production and comes in a reasonable to solder (non BGA) package like a QFP:

Freescale MC68EC000 (68k architecture)
Some various x86 (386 and 486) varieties.
A number of Z80 variates
6502/65C816
A few ARM cores, although most are microcontrollers not standalone.

In some ways the best option is to implement one of the classics (like a MIPS R5000) on an FPGA ( of course that certainly isn’t in a QFP package )

Anything obvious I am missing? Also helpful if they are not crazy expensive, if building say a 64 or 128 node machine.