There was UNIX on Mac desktops before NEXTSTEP. It was called A/UX. It included system 6 or 7 compatibility and was completely posix compliant and used code from System V release 2+ and BSD 4.2/4.3. You could run x11, system 6/7 apps side by side. It had a multi-finder that allowed you to run both at the same time with a unix layer and pre-emptive multi-tasking for UNIX apps. It looked like Mac OS and had the guts of UNIX. It only ran on 68k Macs. The last release was in 1995. (prior to NeXT coming to apple)
I think classic with UNIX might have been more kewl than NEXTSTEP. I'd love to try it.