I don't think Apple will introduce a $199 iPhone. But I think Apple could radically change the rules. Here's how.
1. Use a non-IPS 4.0 inch screen (a compromise between the 4th and 5th generation iPod touch);
2. Use the iPhone 4S's CPU;
3. Equip with 8GB of flash memory;
4. Enclose in a new slim case with metal back;
5. Run full iOS (just like the iPod touch);
6. Leave out the GSM/3G/LTE section;
7. Price at $199 and call it the iPod Flex or iPod touch (6th gen).
Yes, that's correct. Leave out the 3G/GSM/LTE section. However....
8. Offer a new $139 Bluetooth 4.0 wristwatch which includes the GSM/3G/LTE section with a nano-SIM slot. The watch "just works" when near your iPod Flex, iPhone, iPad, and/or Mac. There would also be a $179 version with 8GB of flash instead of 4GB.
9. The "iWatch" (or "iPhone nano") would also have an iPod nano-like screen and iPod functions. It could work on its own if desired. While there'd be a screen-optimized touch keyboard to use in a pinch, Siri (assisted by your "base" unit perhaps) would be preferred.
10. The iWatch/iPhone nano could be worn alternatively as a pendant if desired. (Straps and chains would be accessories, actually.) Or kept in a pocket, like a pocket watch.
11. Don't include third party application support initially, but tell the public you'll get to it. Do include Safari Nano (if possible). Use the same number of pixels as the iPhone 3GS (i.e. 480x320) but in a smaller screen size. The identical resolution but smaller screen (higher pixel density) will make it easier for application developers in the future if they want to support the iPhone nano. (It'd probably be about a 2.5 inch screen, give or take.)
12. Include remote control function for Apple TV.
13. Probably discontinue all existing iPods.
1-7:
You just described the iPod Touch 5th Gen.
The only difference is, that the 5th Gen iPod Touch has exactly the same display as the iPhone 5 - and i can't see why Apple would use a cheaper/worse display in the future, because with the 5th Gen, they probably used the same display for price reasons and because they think people deserve better quality displays.
8. The watch ist the worst possible place to have the 3G/4G module, because there's less room for the necessary battery.
Hell, even the nano SIM slot itself wouldn't fit and leave enough room for the really necessary stuff.
So if there's a base station somewhere on your body, it will be the device you put in your pocket, not the device on your wrist.
Or even a third device, which would only have the 3G/4G module and acts as an access point for both the iPhone (which would effectively become an iPod Touch by then) and the iWatch.
9. Siri yes, keyboard not really
10. Wearing it as a pendant would make it a pain in the ... to use, dangling around.
11. Using the same resolution would be absolutely unnecessary, because at 2 or 2.5" you wouldn't be able to use the device like you use a regular iPhone.
People are buying 5.x inch smartphones because their eyes are so bad that they need huuuge font sizes.
Using a 6pt font or even smaller to display stuff on the iWatch would make it unusable.
12. Sure, why not - but maybe unnecessary, who knows how Siri or whatever is used to control the Apple TV will work.
13. Sure, people would love to be forced to pay 150+ dollars instead of 49$ for an iPod.