While there are rumors of an iPhone mini or a budget iPhone... I think the next logical step would be to go the opposite way... a bigger iPhone.
Most of the Android phones coming out are 4.5" screens are bigger and Samsung has found great success with its big screened Galaxy S3 and Galaxy Note/Note 2 phones, so it would make sense for Apple to get into that market.
While this may oppose what Apple has said about their iPhone being the perfect size... they (or Jobs actually) also said that they would not be making a smaller iPad.
This would also allow them to upcharge for a bigger phone (ie the Apple Tax) and open up several options for their iPhone line. The XL would be their premium/phablet line, the iPhone 5+ their main line and the smaller iPhone 4S form factor would be their budget line. And with LTE becoming standard, a bigger form factor would allow for a bigger battery.
Would you buy a bigger iPhone?
After Math: What's the holdup?
45 minutes ago