In 2007 when Apple unveiled the original iPhone, I was quite surprised they had used OSX as the basis for the operating system. It was notable that they were able use a fully featured operating system on a mobile phone, something that wasn't being done widely at the time. It marked the transition of mobile phones from simple devices to scaled down desktop computers. The iPhone OS as it was then known, seemed very basic given that it didn't support 3rd party applications, multi-tasking or copy and paste, however, it has quickly gained these features, this is one of the hallmarks of a disruptive technology:
"Disruptive innovation is a term used in business and technology literature to describe innovations that improve a product or service in ways that the market does not expect" - Wikipedia
The iPhone was disrupting to some low end uses of laptops by allowing web browsing and email on the move without a laptop (they weren't the first on email, but were on the providing the full web). The iPhone was better at these tasks than a laptop by virtue of being smaller, instant on, with GSM networking built in, GPS built in, no noise, no heat, no viruses and simpler to use.
The iPad has continued this disruption by replacing times when you would use a traditional laptop. In fact, in Technologizer's survey of iPad users, around 70% said they were frequently using their iPad instead of their computer. The iPad has the advantage of having a much longer battery life than a laptop, meaning you don't need to plug it in as often, or at all during normal use. Also, it benefits from a better form factor for web browsing which is better suited to the easy chairs and sofas where people would normally consume written content such books and magazines. The iPad of course has same benefit as the iPhone of being instant wake which typically laptops don't do - at least not well.
I believe that Apple is trying to solve it's own innovators dilemma. Apple's innovators dilemma is that computers are getting cheaper and the primary use for many home users is simply to browse the web and not to use many applications, meaning that any advantage Apple has in it's application base and ease of use, could be supplanted by a web browser only system. This will tend to lead to people buying cheaper, simpler devices that only do just that. Apple's problem that very few people will want to pay $1000+ for a Apple laptop, if the only application they ever use is the web browser. It's notable that none of Apple's iOS devices cost more than that $999 mark that it sells it's cheapest laptop for.
So that's the state of Apple's disruption against traditional desktop operating systems, so what's next?
Both iPhone and iPad already support a larger number of games than the Mac, but what if this could be scaled up? Apple could produce an iOS capable device that plugs into your TV, this would provide the same app store type system to directly compete with Nintendo in casual games as well as providing a media centre for watching online video, music, podcasts etc. Also, I wouldn't rule out Apple making a full TV themselves buying the display panel from someone else as they do with the iMac.
Apple could also produce a version of the iPad in the laptop form factor with a rotatable screen. For this type of device there would be the demand for proper multi-tasking support, Intel CPU and CD/DVD drive support, though of course this support already exists in the underlying operating system, Apple just needs to enable an interface in iOS to make it work.
In a similar vein Apple could introduce a version of the iMac form-factor running iOS. At some point iOS would have to stop depending on desktop PC for initial setup, syncing and upgrades as people would not have such as device in the future.