The Plus replacement is probably coming next year.
They are repeating the first decade.
First the single new phone at 'basic' size. Then they'll add the Plus. And maybe some colour variations. Drop the number like iPad, and simply call it iPhone and iPhone plus. Old LCD gen becomes iPhone Classic (like iPod Classic).
After the current design language is fully kit-ed, they will change the form factor slightly (iPhone 2G -> 3G).
Afterwards, maybe more angular design with even thinner bezels (3G -> 4 -> 5), better screen (Retina intro).