There's a lot of stuff that could be opensourced without any license issues - basically all of the runtime itself. There are codecs and other third-party stuff of course, but there should be enough to get a working Flash Player, just without support for some file formats and such. Maybe Stage3D would have to go as well (parts of it were developed by a third party IIRC, don't know about the license) but that's replacable.
Hell - to the best of my knowledge there isn't even a current open source version of avmplus, which is under a Mozilla license.
Biting the hand that feeds IT
