Mac OS To-Go. What if?
I started off imagining what iOS would look like in classic Mac OS style. I kicked things off stylizing iOS with the platinum design and ultimately decided to meld it with classic Mac OS user interface elements. The project became a mobile version of Mac OS 9 otherwise known by its codename, Sonata. I chose to call the software “Mac OS 9 To-Go” and wanted to see what it would look like on a 90s PDA/smartphone. It led to the first product, the Newton Phone... More here.
The reason Apple did so well in the mobile world is because the company specifically avoided operating systems that tried to act like their desktop counterparts. Microsoft didn't work that out and failed, but BlackBerry and Palm did understand. There has never been a 'computer-like' mobile OS that worked or was successful.