I foresee hordes of Metallica becoming militant Guitar Hero fans.
Anyway, does anyone rember the AC/DC pack released for Rockband? It contained about 30 or so songs, and cost around 20$. It's problably going to be something like that, if they release anything, since Megadeth doesn't have the same mainstream appeal as Metallica, Beatles, Van Halen, etc. to justify a fully priced release.
Also, Endgame is pretty much considered about equal to Rust In Peace by almost every music critic on and of the internet.