It could become a mess. Not as a mess as the Samsung line-up, but it could become much more complicated.
Now we have:
The last model: iPhone 5S - 650$
Last year model: iPhone 5C - 550$
Two years ago model: iPhone 4S - 450$
Very clean, very clear line up, even if the 5C is technically not last year model, but that's how Apple has been doing for a long time now.
Now if we get two more screen size, how will the line up look like? It will strongly depend...