If you could make a non-linux OS go mainstream, which one would you pick?
I make the specification of non-linux because otherwise this would just become a thread full of obscure distros that do the same thing as a million other distros.
Some lesser known OSs:
AROS - based on Amiga OS, has some derivatives like IcarOS and MorphOS
To name something that hasn't been mentioned yet, ArcaOS, which based on OS/2. It supports modern hardware and in addition to some preinstalled software, it also has some compatibility layers to run software from other OSs.
I still remember the huge marketing push IBM put into it in the mid-90s. Who would have thought it wouldn't take off when they never actually showed what it looked like. Just a bunch of people describing it.