As you said (and I was thinking about this just as you were typing the other reply you had, how funny! lol) maybe the portal thing and the Elves being interested in Earth's growing tech is why they are there and trying to get Jane, so I'm going with that idea.
and this is just really my personal speculation,
people, based on a few spoilers and what happens in the comics. So... no guarantees...
Jane is on Earth doing research into portals with Erik and maybe Darcy is there too.
Malekith has Elves on Earth in human form keeping an eye on things, in particular things related to making portals.
Malekith learns that Jane and Erik are close to or now able to do this, and he decides to send someone in to kidnap Jane and/or Erik and maybe Darcy gets mixed up in this as well.
Heimdall has his work cut out for him keeping an eye on the realms right now, but he promised Thor to keep an eye on Jane and so he tries to do that some too. He sees something is going on, and Jane may be in danger from the Dark Elves. Thor finds out and gets Odin to send him down again to help (maybe Odin doesnt want them to have that power either, so that's a reason for Odin to do that). Thor goes down and retrieves Jane at least bringing her back to Asgard for safety, and maybe Erik and Darcy tag along as well for some reason.
Asgard (Nornheim) is attacked by Hela's army
Thor goes off to fight in Nornheim with the Warriors 3 and Sif.
In the meantime, Loki is in prison, Frigga talks to Loki and seems to get through to him some that his family does love him, frost giant or not.
Thor and the Asgardians have a hard time, A LOT of losses. they have to pull back. Odin uses the Odinforce to *cough McGuffin cough* put up some kind of forcefield up or keep them back from invading the heart of Asgard. But this temporary and takes A LOT out of him. THis is the reason why he can do very little else and is not really being mentioned in spoilers. And the reason he cant travel to Valhallah. He tells Thor to go make nice with his brother who knows how to get to Valhallah with his magic because there is no way for him to do that right now.
Loki seeing he might have an opportunity to get out of trouble faster and agrees. Jane and co possibly go along with them.
Loki and Thor make nice some on their excursion. Thor and Jane get to know each other a bit and he seems to be more in love with her. They get reinforcements with the dead Valallah warriors and are on their way back to Odin's palace.
Odin is weakening. Surtur and Hela know this.
On the way back (and this is the part that you won't like AM). it is revealed that one of the 3 humans, is not a human at all but a Dark Elf. I am going with Jane as that would be the biggest knife twist. It goes along with the Lorelei storyline, and also goes along with the possession by something associated with the Dark Elves spoiler. Thor would be heartbroken and it would also be revealed in the course of this that Malekith has Jane (and possibly the other two if they all are Dark Elves) and that Malekith is planning to have them build a portal from Svartalfheim to Midgard in order to have his army come and take over Midgard. Thor is furious and upset, etc etc... Loki and probably Warriors 3 are with them they all have the reinforcement army, so Thor believes he can go to Midgard and protect Midgard and save Jane. He asks Loki to send him to Midgard, after some argument about where he is needed more (would be good if this was more from Sif and the Warriors 3 than Loki)... after a bit Loki agrees to do this for his brother, as a sort of peace offering, or to get him out of the way whichever you prefer.
Hela's ship rams through Odin's barrier and into the palace and her army invades. Surtur also shows up and destroys stuff.
Thor lands on Earth and starts having a big smackdown with Malekith.
Loki Sif, etc, and the army all arrive at Odin's palace and start fighting. Odin and Loki have a moment of reconciliation when he realizes that Loki is there and Thor is not. At some point in this we have that iconic moment that jaqua99 mentioned before, but as I pictured it earlier with cuts between the Odin, then Thor on Earth, then Loki.
Thor and Malekith go at it, at some point Malekith gets desperate and is losing. Surtur steps in and makes one of his warriors into Kurse to battle Thor. Surtur then appears at Odin's palace and at some point and starts fighting Odin specifically. Frigga is still alive possibly in this scenario and is battling Hela.
Thor vs Kurse... nuff said. The real Jane and co are saved, Earth is saved. Malekith is killed, Kurse or more likely I think back to Algrim becomes the new leader of the elves, and peaceful now. All is ok on earth. Jane opens a portal for Thor to get back to Asgard to help there.
Odin and Surtur take each other out. Loki is upset. Thor gets back too late and blames himself. But they still have Hela to deal with, this is where Hela possibly kills Frigga, with a new rage in both of them, for their father's death "For Odin" (as Jaimie Alexander hashtagged not long ago, interestingly #ForOdin... ) They find a new fire in them and they beat her and her armies butts back to Niffleheim with the culmination being the big Icelantic battle which is set on Niffleheim.(again I'm guessing that the report that Loki and Thor were on different sides there, might have been assumed incorrectly).
Hela wont mess with them again, or will she?
in the end, the Asgardians have more respect for Loki for being there when he was needed most, and lost respect for Thor for going to Midgard, and Thor blames himself for Odin's death as well. So in the end we wind up with Thor going back to Earth, very upset and in turmoil, maybe with Jane, maybe not, and we have potentially Loki on the throne of Asgard for future films.
Adding: and of course the end could easily be changed to Thor gets back in time, and doesn't lose any Asgardian respect, and is given the throne, but I just think it's more interesting the other way for future films. LOL For one thing, like I said, it's an excuse to have him on Earth for Avengers 2.
I am having trouble fitting in Frigga's death here, as you may have guessed, I thought it might be cool to have Frigga fighting Hela at some point, either with her dying right after Odin's death or earlier in the film to give Loki added reason to go along with Thor, but it's tricking fitting it all in. and of course there is the worry if she winds up on Valhalla or Hel and them seeing her again, etc. And maybe it is only Odin that is dying not Frigga, and that spoiler was a mislead by the studio. Dont know yet. Like I said this is A LOT of guessing!
Alternately, Jane and co stay behind during Thor and Loki's trek to Valhallah and Dark Elf Jane kills Frigga when Frigga catches onto what she is. Thor gets back with Loki etc with reinforcements, and finds Frigga dead or dying, and they get the elf and they find out what Malekith is up to and goes to defend Midgard then (and also get further revenge on Malekith for Frigga), and again Loki helps him get back to Midgard. SO there's another scenario...
adding: as for Tyr, like I said maybe he's Hela's right hand man (pun intended
adding: at some point there possibly is a flashback involving Algrim, Malekith, Odin and Frigga, that shows why Malekith is so vengeful, possibly Thor or Odin caused his half dead status in the past.