Apple should just call the next iPhone the "iPhone".
After all, Apple isn't calling this year's Macbook Air the "Macbook Air 3", are they? The iPod touch is still the "iPod touch".
Even if Apple does decide to give the iPhone a progressive numbering scheme with each passing year, they have to realize that at some point, it would sounds kinda lame. Apple isn't about lame.
I'm not going out on a limb here. Apple has indicated that it wants to focus on what the experience, not how many cores or how fast the iPhone is. And calling the next iPhone simply the "iPHone" would do that.