Ok, so I'll take you up on the challenge with MINT: Gem and Tos weren't really developed much by Atari, but they did buy in MINT which has preemptive multi tasking and memory protection. With the Aes being retargetable (graphic cards), gdos vector fonts and postscript printing, tcp and lan networking stacks, shell, global shortcut system, posix compatibility and multi-user capabilities, it managed to evolve Tos to something quite modern!

https://en.m.wikipedia.org/wiki/MiNT