The current Galaxy S4 - full hardware spec, removable battery, micro SD slot and all; but stripped of TouchWiz and Samsung's other customisations, running AOSP - pure Nexus level Android!
I imagine if this arrives it will be something of a killer device - all the benefits of a Samsung and all the benefits of a Nexus in a single device, you've just got to love it.
I'd also expect that removing the Samsung extensions will free up a lot of space in the built-in memory - how much remains to be seen, we may be surprised one way or the other.
If this does arrive, and my hope is that it does, this could turn out to be another boost for the incredibly popular Galaxy line. And that may be the biggest reason why it doesn't happen - much has been made of Google's concerns about Samsung dominating the Android market. This doesn't seem like the sort of move they would make if that were true.
Either way, we should find out tomorrow. At least the wait isn't too long...