Would be interesting to make a 32 bit Apple 2 style computer. Include a ROM for a means to boot, and leave everything else simple, with some nice slots. Could be a great development / learning machine.
One of the bigger challenges is integrating peripherals. I got bogged down trying to do SD Card interfacing. There are off the shelf bits of IP from Xilinx, etc. you can use to do this, but that sort of defeats the purpose of the exercise.
I think modern machines started their slide into mind boggling complexity when bus speed and CPU speed outstripped RAM speed. So much complexity and unpredictability is in the all the infrastructure built around cache.
Something like an Amiga or Atari ST was still not hard to understand all/most of, despite being 16/32 bits.
I have a couple of those. The HYDRA was a lot of fun.
A search of the interned didn't turn up what I was looking for, but I'm very new at hardware work. Perhaps newer chips have such tight timing requirements that you can't work with them without using a SOC?
If you're just looking to breadboard up a computer but don't want to go back to 8-bit processors, the Motorola MC68000 / MC68008 used in the original Apple Macintosh is a 32-bit processor in a DIP package running at a manageably low frequency and can be found on eBay inexpensively.
No. The busses used to access peripherals and memory are not suitable for off-die use. (This goes for all ARM cores, not just Cortex-M0.)