Back to the USB 3.0 vs SoCs thingie - who says that it has to be within the SoC? You can buy a cheapo SoC and slam next to it a USB 3.0 hub (provided, of course, that the SoC exposes the internal bus and can talk via GPIO with other components, and most of them do) - I've already mentioned SMSC as a provider of quite cheap yet reliable ICs for all kinds of things, and of course they have a USB3.0 hub as well. So, slam it on instead of waiting for a cheap enough SoC that comes with it, and your problem is solved. Hardkernel for example uses their controllers instead of relying on the anorexic USB facilities of the Exynos 4.

That being said, I really see no reason why a phone would have USB 3.0 facilities as even the fastest eMMCs are slower than USB 2.0 in optimal conditions. If you want to push graphics through it, HDMI is by far a better solution, and then there is Thunderbolt... Either way, unless you want to have the OH as a docking station to which you connect other peripherals, USB2.0 in a proper dual host/OTG mode is more than enough for pretty much anything you'd want to interface with.

Of course, if you're thinking of extending the core specs via the OH, then none of these interfaces are good enough - you'd have to expose the GPIO and VBUS at the very least, and even then you won't be able to affect the internally packed SoC's limitations all that much.