Although i am a huge Twilight fan and a huge fan of Joss Whedon shows, i dont think mixing the two is a good idea. There are many Joss shows to get guests from so i dont see why it needs to be mixed and like its been said, Twilight itself has its own cons already. Like Sazzy said the Twilight cons sell out super fast, which wont leave much for the Joss fans ticket wise. I think stick to what it originally is, just a Joss con.