Not much would happen. Big chunks of what constitutes OS X is already open source. Nobody runs GNU/Darwin the same way nobody runs Plan 9.<p>Apple's market is primarily geared towards people who aren't interested in the tech and are paying for the logo. After all, anybody who wants an OS X equivalent system who knows what they're doing can buy a bog-standard PC and run some free unix on it with GNUStep. So, the licensing for Apple products doesn't matter because Apple's market excludes anybody who cares too much about licensing (or anything other than branding).
The parts of OSX I'd like to see open-sourced are exactly the parts Apple will never open up. I'd love for their desktop environment to be open-source - if I could have that running on top of, say, Arch, I'd be incredibly happy. But the desktop environment is one of the big things Apple uses to sell their computers, and it wouldn't make sense to give it away like that.
From Apple's perspective, the whole point of the work they put into macOS is to add value to macs, the selling of which is their bread and butter. (or used to be ... i guess now it's mostly selling iPhones.) If they were to open-source it, it would be quickly modified to run on bog-standard PCs, removing Apple's motivation to put any more effort into it.<p>In other words: don't hold your breath.
Apple did open source Darwin years ago:
<a href="https://en.wikipedia.org/wiki/Darwin_%28operating_system%29" rel="nofollow">https://en.wikipedia.org/wiki/Darwin_%28operating_system%29</a>
<a href="http://www.puredarwin.org/" rel="nofollow">http://www.puredarwin.org/</a><p>There is one implication that Apple is currently rewriting kernel level that will need a huge effort and was discussed in HN.
Being that chunks of it are already available via <a href="https://opensource.apple.com/" rel="nofollow">https://opensource.apple.com/</a> not much would happen.<p>The NextBSD project was consuming some of the open sourced bits and bolting them onto a FreeBSD fork, but I'm not sure where that project has left off.