But the thing is, many of the major flaws of WordPress are baked into its theme and plugin formats. Much like with Excel, you'll need to match it quirk-for-quirk to interoperate with it. And what's the point of simply reimplementing WordPress?
I think you could probably get away with shooting for something like 70-80% functionality via a wrapper layer around your new APIs. Even Excel and Word aren't 100% compatible between their own versions, and WP is going through a transition period where many older plugins don't work either.
The goal would not be to reimplement all of wordpress, but implement a compatibility layer suitable to emulating enough of the basics to allow some migrations and encourage people to try out something new.
There's dozens of CMS and blogging engines out there, but no one has attempted a WP-emulation layer (as far as I can tell). Although, given it's PHP nature, and direct grabbing of $_GET and $_POST and such, it may simply not be possible at all.
