I don't think Sif will die, but I will love if she sacrifices her "love" for Thor to keep his relationship with Jane and Earth in tact. As much as people dispute it, Jane is Thor's only real reason to fight for Earth, even if he is friends with Coulson and Selvig among the other Avengers. Jane is the no.1 in his eyes as far as we know so far and I like that development, even if it is a stray from the comics
I think Sif sacrifices her love for Thor to be with Jane cuz if u see the interview with Jaimie she say's "...Sacrifices Her World...". For example:
In the comics Sif joins her life force with Jane to bring her back to life and so forth... but i think in the film she might do the same and save Jane (from the Malekith encounter from the trailer). And the scene with Greenwich comes at the end and Jane is still alive so maybe Thor brings her back to Earth to be reunited with Erik and Darcy but before that, I think Sif already sacrifices a lot and they probably.... if the rumors are true, had Frigga's funeral (Thor looking sad and angry at the same time and Jane is by his side and people behind them) cuz the scene with Greenwich comes at the end because the ship in the trailer looks so big and dangerous and that kind comes at the near end of the film and even the producer or someone from the film said "The film starts on Earth and ends on Earth"
so Jane is still alive cuz she's wearing something different like red boots and from the trailer when Thor transport her, she's wearing a scarf and normal boots so they switch the scenes to confuse you to not no what's happening.
But yes the film obviously will explore the Thor/Sif relationship not just cuz of the comics but cuz at the end of the first film she clearly has feelings for him. And in general they want to make comicbook fans happy with exploring their relationship and even the ones who haven't read the comics but ship them anyway. But from the first film i got a brother/sister vibe from them and i separated them from the comics (like what if they were not canon) but they do need to explain Sif's feelings though anyway. I think Thor/Sif would be boring to be honest cuz they're both warriors so they're similar in a way and would suit each other but.....
I really like Thor/Jane because they're different and some people might think they don't fit but they could, by good writing and chemistry which for me i felt in the first film but not too much which yes the relationship between them was forced and fast but in the 2nd one they're expanding on their relationship which i like and yes in the comics they were together so it's not like the writers created a fake character for the film and added a fake relationship with Thor. It's the differences between them that makes them more interesting and fun
. Heck! alot of people liked Darcy and she's fake created but u no there are some people who dont like that but u cant please everyone.
Jane doesn't deserve the hate she's getting cuz plz try to separate the actor from the character
cuz that's wat i did and she really doesn't deserve it. She's an amazing character in her own way. She doesnt have to be a warrior or kick butt like Sif for fans to appreciate her. In the comics her memory got wiped cuz she was not worthy of Thor and was taken back to Earth, How horrible is that? and some people are encouraging for that to happen... is beyond me.Who knows, she might kick butt and fight in this but she's her own person and u really don't need to compare her to Sif.
The amazing spiderman explored Peter/gwen relationship and they weren't the main canon ship but alots of comicbook and non comicbook fans liked them as a couple so why not for Thor/Jane? I think when Jane gets to Asgard, Odin is obviously gonna have a problem and say she's not worthy but that's what makes it interesting and it really taps into the Thor/Jane relationship more.