Hmm. I think Emil would initially want to be the designated hero (Raoul).
But as soon as he realizes the Phantom gets a lot more songs and fandom, plus getting to order around all the other characters and cause destruction, he'd demand to play the masked monster genius. Kind of like Bottom the Weaver in "A Midsummer Night's Dream," who wanted to play all the kewl parts in the Pyramus & Thisbe sketch.
P.S. I'm loving the idea of Lalli as the ballet girl Meg. All the fangirls would squee. And Siv, the Voice of Doom, would make a perfect Mme. Giry.