It should be a sin to even offer an 8GB model and the 16GB model is quickly becoming a joke. A while ago on here me and a couple of others had a discussion about the starting sizes and we did the math and found that 16GB has been the size thats been sort of "stuck" in comparison to standard memory size growth, especially with retina displays and what not which means apps and OS's become bigger. For example the MacBook Air used to start at 64GB and as time moves on space requirements do as well (coupled with cheaper flash memory sources) the iPad, MacBook Air and MacBook Pro's seam to be expanding in memory size while the iPhone doesn't. Also 16GB's really means closer to 12GB's and 8GB's means closer to....well its very small! 32GB's seams to be a proper amount of space given how much space iOS takes up along with modern apps. My iPhone 4 16GB is so slow even with 1GB left and I'm not even on iOS 7, my dad got screwed with the 8GB iPhone 4 and iOS 7...its the slowest phone ever...if he had to make an emergency call it might take some time...I just ordered an iPhone 5c in 32GB's, I would have liked the 5S but $300 for 32GB's was steep for this month and I had waited too long already (my iPhone 4 won't charge, is slow, camera is screwy, etc.).
32GB iPhone 6 as the starting point would make most sense, followed by a 64GB and possibly a 128GB...I got an iPhone for the iPod and the phone being in one and I'm sure that's a selling point for allot of people. I'd say compromise with a 24GB iPhone but that wouldn't be in Apple style and even though its technically a logical size between 16 and 32 it might be weird. For the free iPhone or the lowest possible one (likely a 5 or 5S) 16GB would be fine but for the flagship model 32GB's and up should absolutely be standard...for a phone that doesn't have a microSD card slot getting stuck with 16GB's sure is a pain and as apps get more advanced with more graphics it'll be a pain that keeps on hurting. I tend to hang onto my iPhones for a long time too so having allot space on them is nice since in 4-5 years 32GB's might very well be a normal size if not small! I imagine if they made a 64GB iPhone 5c I would have bit the bullet on that.