You know, there's something I noticed quite a while ago-
The deal they have right now, where Shiori is supposed to kill her the moment she finds true happiness, is pretty much the same as the original Faustian bargain:
When thus I hail the Moment flying:
“Ah, still delay—thou art so fair!”
Then bind me in thy bonds undying,
My final ruin then declare!
Then let the death-bell chime the token.
Then art thou from thy service free!
The clock may stop, the hand be broken,
Then Time be finished unto me!
I'm not sure if this is intentional, but I do quite like it, especially because it's contextually the reverse from Faust, where of course Faust was the one who wanted for himself to achieve happiness, and Mephisto was the one who wanted to end his life.