1.i think Elsee's sister is a vintage...ELsee mentioned that she's a great demon.. maybe like she will become a mad demon and vintage ask for her to join the organization OR due to her greatness,she was forced by vintage to become a member by using a magic on her..
2.i have a hunch also that somehow Keima's father is related to demons or whatever... maybe he's a buddy of someone just like Yuri nikaido(keima's sensei who happens to be a demon with akari as a buddy)
HEaven--->>> divine beings/ goddesses
New Hell-->> new demons
OLd Hell-->>> runaway spirit/weiss
3.kinda thinking that human realm is kinda like to FULLBRINGS. where they can either awake a devils/goddess power due to their actions
4.MY mind also has a thought that there would be a counter-part of goddesses in weiss version and Keima's counterpart in human realm that can awakens weiss's power.
5. if no.4 is true im thinking that vintage true mission is to awaken the most devilish weiss but to do that they must kill the goddess so no one can seal the weiss anymore
6. if 4 and 5 happen it will become a serious-fighting-manga! hope will not happen coz i prefer reading/watching comedy-romance-harem manga/anime...
and CURiosity never stops :D