It would be EPIC, imo. The game takes you from the start with Metallica opening with Kill 'Em All and Dave breaking away to form Megadeth. As the story progresses, Peace Sells is sold, breaking Megadeth into fame. Finally it takes you into their final album (the next one).
Imagine using 20 different players through the game. Wow. But I have the soultion: For every album, the band members that contributed are changed in the game.
There would be two lead guitars, a base, drums, and vocals. I'd buy it in a split second.