Instead of placing the battery inside the handset, make a handset that just has a connector on it (wouldn't have to be a bulky, thick connecter like the USB series, could be done in any number of ways, including contacts on the back.
Open up the design, then let case manufacturers include batteries in their cases, since people overwhelmingly use cases anyway. Now the phone is very thin, so the case can be thicker to accommodate a battery.
Consumers needing long, long battery life can choose a wacky big case. Consumers needing very little battery life can choose a case with a battery that gets them close to current thickness levels. Need a new battery? Replace your $60 case instead of your $700 phone. Going on vacation? Get a fat silicone case with a fat, fat battery in it, just for the trip.