This is a great question, however only appeals to a limited audience. I believe Apple will not release a 128GB version with the iPhone 5S, but will offer that storage option in the following version of the iPhone. I believe threw following their current pattern in product releases the addition of the storage just doesn't make sense until the iPhone 6. Just my honest opinion.
Especially if it's as slow as the current iPhone SSD's are. The major use of my iPhone 4S is photos & videos - working with large 8-megapixel files & HD video files, as well as transferring over USB 2.0, is just plain slow a lot. I think a larger/faster drive + USB 3.0 would be awesome. Especially USB 3.0 - I usually dump my photos/videos to my computer when I hit around 2,000 items in my Photo app and it takes forever to transfer to my computer, which is usually a couple times a month.
I would also really like to see the new 802.11ac on there, not just for PC syncing, but also for iOS device to device syncing, plus 128gb & 256gb hi-speed SSD options for the iPad. There's such great software available, like Avid video editing for iPad, that you can really do a lot with, but you're limited by the transfer speeds & storage capacities of the devices.