At the end of the Agents of S.H.I.E.L.D. episode "Yes Men" in which she appeared on Earth, Sif said of Lorelei that "Odin ordered her back alive". At that point, Odin wasn't on Asgard's throne - Loki was, but he was indeed posing as Odin. This means that it was actually Loki who ordered that Lorelei be returned to Asgard alive - and that would suggest that the mischievous adopted brother of Thor must have some use for her. As Loki will likely be playing a key part in manipulating the onset of Ragnarok, he could surely use the manipulative powers of Lorelei to do so. Then there's Hela. In the comic books, she conspired with Loki to bring about Ragnarok and, given what is expected to happen to Thor in Avengers: Age of Ultron, expect her to have some part to play in the Marvel Cinematic Universe version of the apocalyptic war as well. Hela is the ruler of the realms of Hel and Niffleheim - the places Asgardians go when they die - and, given that Thor may well be there when Thor: Ragnarok begins, she may initially ensure that he stays there at the request of Loki so that Ragnarok can occur unopposed (prior to Loki suddenly deciding that he actually needs Thor to save his own skin when things go wrong). She could also provide Loki with some of the enemies of Asgard that he requires to start the war - perhaps by unleashing them from Hel itself. Having two deadly women on his side could make Loki's wishes come true in this movie.
