There's far too much already. It's not what's left to put
into a phone, but rather what ways can your phone become less of a ball-and-chain. Just because you choose to carry it around all the time and choose to pay monthly fees for it regardless of if you use it or not, it doesn't mean that it
should be this way

The key differentiators I believe we will see have to do with competing at the contract/service level, not simply trying endlessly to cram more ridiculous amounts of features into a small space. It's already getting challenging to dramatically differentiate anything in this space. Aside from n+ megapixels, n+ memory, n+ battery life, n+ bandwidth, n+ etc. etc., there's really nothing novel that we've seen for many years. A touchscreen? Yawn at best and boo for those who prefer the tactile feel of buttons, pads, trackballs and keys.
But it's a great question you pose since the world seems unable to imagine a concept of
added value that doesn't directly involve added or expanded features. Maybe someday there will be no Logitech Harmony Remotes, no garage door openers, no library cards to scan, no credit cards to swipe for purchases, no autonomous sprinkler system, etc. and instead all of these functions and more will be commanded from our smart-mobile devices of choice. Bringing more and more pre-packaged and easily configured tools will probably continue to evolve, much like software in general.
But I think those businesses who cling to the cheap-entry, expensive long term commitment contract approach will find themselves losing a battle with Darwin or experiencing "random" mutation for survival

All we really want from AT&T are reasonable rates and good service. iPhone/iPad users expect to pay more for everything tech and expect great service to match: reliable and as fast as possible. I'd bet if you reduced 20%-50% of the features in the iPhone yet truly provided blistering fast data speeds, crystal clear calls that never drop, from anywhere "reasonable" you would expect this to be the case, that the market would shift. I believe the discussion will eventually shift to the real and numerous aspects of value from the consumer's perspective, and I believe the market will eventually follow.