Well, you could play as some intergalactic parasite that feeds off stars and planets. A sort of Galactus meets space Chtulhu, but I think it would get boring fast.
Or you could play as an alternative universe (non-Star Wars) version of Darth Vader and the Death Star.
But really, if you have the power to unleash annihilation on such a scale...what opposition, gameplay-wise, could be fun/ challenging? And wouldn't it be more fun to play as the underdog/ rebel force? You know, to beat tremendous odds?
What I'm saying is, what sense of achievement/ accomplishment can there be in a game in which you are such a force of destruction?
"Magpie"-ish considerations aside, what would be the motivation for the player to continue destroying? What's the point if there is no "weight" to it?