iOS for iDevices, Mac OS for the Mac. The Mac's operating system should be called Mac OS (11, 12, 13,..), it's that easy.
A closer look at Apple's nomenclature reveals that their products thought for the consumer have the 'i' at the beginning of the name (iPhoto, iMovie etc.), while the products built for the pros have an 'X' at the end of their name (Final Cut Pro X, Logic Pro X etc).
But Mac is something in between - see the MacBook Air as the Mac for the consumer, while the Mac Pro is the pro's machine. So calling it OS X seems too 'pro', at least for MacBook Air and iMac users. MacBook Pro and Mac Pro users may see it differently.
Additionally to being called Mac OS 11, i wish the next version of the Mac's operating system would have these features:
• iOS-inspired UI
• Music app
• Movies app
• TV shows app
• Photos app
• Newsstand app
• iTunes split into 5 individual apps
- iTunes Store
- Music
- Movies
- TV shows
- iSync (revive it, i miss it!)
• Touch ID
• Control Center
• AirDrop from Mac to iOS/iOS to Mac