Well, so you know how Dr. Erik Selvig has the cosmic cube and Loki has him under mind control, maybe the cube is like a bifrost like someone said and Loki uses it to make a portal into Asgard so Thor can return. Like in the comics Loki wanted to challenge Thor and defeat him. Another idea I have is, Thor's enchanted hammer Mjolnir can create portals to other dimensions! Well that's all I have in the think tank so keep in touch!!!