How could Apple merge OS X with iOS?

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With the recent news/rumors of Siri and Maps coming to OSX in 10.9 it's becoming more and more clear that they are bringing the two OS's together, possibly to ultimately merge them at some point. I wrote about it in the forums last week (The Future of Apple Products), when the news of Apple possibly switching to ARM chips on Macs in the future. So with these two things hinting of what's to how would they implement it that would make it function cohesively while keeping Macs fully functional computers and not making the mobile OS bloated? And what would the main UI look like?

I haven't given this too much thought (hence me asking the community for ideas) but I can't really come up with anything that works in my mind yet. Would Apple do something similar to what Microsoft has done with Windows 8 and make the UI visually similar on both Macs and iDevices or will they still continue to give two distinct UIs but allowing the software to work on both? Would this take a full transition to ARM in order to work?

Everyone is kind of a assuming that OS Xi will be when the actual merger will occur, if it does happen. Granted Apple could go OS X 10.10, 10.11 etc. until Xi is ready so the timeline could be still a few years away (maybe until ARM is ready). I was also thinking, maybe they wait to fully overhaul iOS until they are ready to release Xi and just release them both at the same time. Maybe by the time iOS 8 or iOS 9 is due.

Who knows what Apple is cooking up but I'm sure there is a road map at least 2-3 years ahead from what's currently at retail in Apple's labs. Software takes a while to develop, by the time one version is released the other version is already being worked on. (Which is why I don't expect any radical changes in iOS 7 with the departure of Scott Forstall look some people are throwing around.)

Have any ideas? Crazy UI concepts? Post them up. I'd love to see.