If Steve Jobs steps down then Jony Ive for CEO!
The "E" in "CEO" stands for 'executive', ie the one responsible for executing the business plan. There is no one on the planet proven better at that then Tim Cook.
The fact is that Jobs has always been terrible as an executive. His goodies have been pulled out of the fire time and time again by the great people he inspires to be around him.
The Ives, Schiller, et al braintrust will carry on the tradition quite well.
The one thing that will be hard to replace in Jobs will be his ability/willingness to prune dead branches and to send things back to the drawing board if they aren't perfect. I don't see anyone else who can turn the troops on a dime without questioning they way he can. Don't know if its fear or respect, but at the end of the day, what's the difference.
Steve's greatest product since his return from the wilderness hasn't been iPod, iPhone, iMac or iPad.
Its been "iApple". THAT'S his baby, and there's no way he's going to let it die the way it almost did last time, particularly now that he undoubtedly has a real sense of his own mortality.