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
It really feels like any alternative in mobile space would be a welcome addition, though first we'd need OEMs to stop being assholes and allow users to more easily install custom ROMs or whatever.