Just FYI, I've been looking to somewhat start splitting CoreBase and not requiring -base anymore. CoreFoundation is lower level than Foundation so it's very hard, and even impossible with the public API, to implement certain aspects (like CFDictionary and CFArray callbacks). I'd also like to see where I can take it. At the moment, I've been (slowly) implementing CFString using libicu, most of it is simply wrapping CFString functions around libicu functions and types.
Anyway, since the question has came up I figured I'd make my plans public. This will take some time due to my lack of time, but at this point is a "goal" of mine.