Okay, Thor fans are going to hate me for saying this, but I had his idea last night.
DISCLAIMER: My knowledge of Thor comes solely from the MCU films and a few off-hand issues of The Avengers and an episode or two of that Avengers cartoon I have caught over the years.
Portman wants a better part and is honestly better than the role of Jane. Fans want Thor to be with Sif. How about instead of writing Jane out simply as "We come from different worlds" just as the trailer hints, something bad BESIDES DEATH happens to her?
I doubt Marvel would do this at it takes too much creative effort to invest in a character, but what if instead of that guy killing her, he uses magic to turn her into a villain. And not in a "oh, we must find this dues ex machina to turn her back." I mean a clearly irreversible thing that turns her evil. Hell, why not she becomes Echantress? Maybe Loki does it just to spite Thor?
That way, she is a reminder of Thor's greatest failing. She can then appear randomly in future films as a cause of pain and shame for Thor. Basically kind of like Harvey Dent is for Bruce Wayne.
A random thought, but it would be more interesting than just sending her back to Earth with a hug or killing her off. Just a thought.