If I were to build a system with lots of 6502s, I think I would build something transputer-like, so that each CPU has its own memory, and the total system can have more than 64 kB of RAM.
The alternative of a massively parallel system with shared memory, on 6502, would mean all CPUs have 64 kB or memory, combined. I think that will limit what you can do with such a system too much.
These were intended to get to 4k cores (but 32 bit), w/32K on-die memory, but the ability to read from/write to other cores. A 6502 inspired variant would be cool, but to not get killed on the cycle cost of triggering reads/writes you'd probably want a modified core, or maybe a 65C816 (16-bit mostly 6502 compatible).
The alternative of a massively parallel system with shared memory, on 6502, would mean all CPUs have 64 kB or memory, combined. I think that will limit what you can do with such a system too much.