Re endianness, I confess I hadn't even thought of that aspect of it. (I understand that Cortex-A processors can run in big-endian mode but I bet it's not that easy.) And I bet there are a million other things that would be hard to make work.
But I still wish EmuTOS/FreeMiNT could be made to run on a modern CPU. It makes me sad to think that the 680x0 architecture basically isn't going to go much further than the Apollo 68080 (or Coldfire...), whereas a modern ARM (or X86 or even RISC-V...) chip is stupendously more powerful and I'd love to be able to run those OSes on that. Oh well, I can dream I guess...