i was thinking about re-writing the bluetooth daemon, in order to...
The NIH is strong with this one.
IMO you'd be better off putting that enthusiasm into fixing BlueZ - you might actually be able to fix some real issues and improve things for a great number of users relatively quickly.
Writing a new, competing, piece of software is going to take a while to achieve both feature parity and see any adoption by major distros.
retro-compatible (exposes the same D-Bus APIs as BlueZ)
Is there any reason for this? I can't think of anything off the top of my head that would require it. It's an admirable goal but make sure it's worthwhile doing this and that there aren't actual benefits that could be achieved by breaking compatibility.