Something Unix-like (i.e. solid "modern" OS) compatible with classic MacOS apps and such, with our beloved GUI. Kind of like what OS X would have been if they'd gotten started 10 years earlier, or what A/UX nearly was, or what Copland might have been if it hadn't drifted aimlessly into oblivion.
The idea was discussed several years ago in the A/UX newsgroup, and got a name (NouveAUX) but no farther than that.