There's A LOT of stuff in the Norse Mythology in our myths, that I doubt very much is in Erik's book or will ever be shown as being true in the MCU (how Odin got his 8 legged horse, being one of those myths
). Although I would find it rather amusing if the mythology surrounding Loki especially was there, and there were different reasons for certain things the mortals based it on. Like Loki in the MCU created his children "magically" not the ways described in the myths, but the myths that are written about him on Midgard is just some ancient mortals, annoyed with his past mischief, talking smack about him. LOL
Oh, but how I would love to see Sigyn introduced (only if done right! with an actress who can act up to par with Tom!) Doubt that'll happen though.
I can see some mention of Sif and Thor being a past item, but I think it seems more like they may have - her feelings at least - start to show more in Thor 2, it seems like their is a hint of more than friends interest on her part by the end of Thor 1.