My main complaint with Thor: The Dark World as a movie was the edits/cuts/pacing of the first act. Given that, here's how I would fix it:
First, postpone the LOTR style opening, and have the movie begin instead with Loki being brought before Odin. As Odin talks about the damage Loki's schemes have caused, the visuals show Heimdall repairing the Bifrost with the Tesseract. End the Loki-Odin scene by establishing Loki's jealousy of Thor:
Loki: And what of Thor? Is he to be given a hero's welcome yet again?
Odin: Your brother has been sent to fight...
Which flows into Thor battling with Sif and the Warriors Three against the Kronan. However, this is not the beginning and the end of the "policing the realm" arc like in the movie. Instead we cut-watch the group travel/battle through the different parts of Asgard, Vanaheim, and other realms, with Thor learning that not everyone appreciates the rule of Odin (an idea I admit I have shamelessly stolen from another forum member's proposed changes that I really liked). Thor now understands that protecting people requires the ability to act without the burden of a crown, giving some meaning to his return to Earth at the end of the movie.
More importantly, however, the new scenes establish the growing battle-weariness of the Asgardian army, and their losses from such a lengthy campaign. They worry about the other enemies of Asgard causing trouble:
Sif (or whoever): The Jotuns are still recovering from the destruction of much of their home, and the fire demons of Surtur have been strangely quiet recently, so Asgard is safe at this time.
Volstagg: Then let us continue with our task, and get to the food and merriment all the sooner, for I have developed a large appetite!
The map-hopping of this arc can then hop along into Svartalfheim, and we see Malekith's awakening. Mal revives the remaining members of his race, and then awakens their hatred by recounting the previous Bor-Malekith war (with some of the appropriate opening LOTR visuals, but with an Eccleston voiceover). [Note: I didn't really mind the actual LOTR style opening per se, but by the end of the movie it felt like it was lost amongst all the other opening plots being established and dealt with, so I would try sectioning it out when it would have more impact emotionally.] With this intro for the dark elves, a suddenly vulnerable Asgard is now seen to be facing a serious threat. The Convergence is established by Mal as happening relatively distant in the future (i.e. several months), so he commands the dark elves to prepare for war, and to begin searching in secret for the aether. [And the audience gets to see him actually be a ruler of his people.]
Cut to Thor, Sif, and the Warriors Three. They are finally ending their long campaign, and Hogun can now return home without it feeling too jarring. This "victory" feast can play almost exactly like the movie, with Thor feeling apart from the celebration. The only change: because of the extra scene time Sif has had battling at Thor's side, she can now reference it and how she wants that camaraderie to become something more. Thor can clearly establish his desire to return to Jane and Earth, and Sif's true feelings will finally be expressed to Thor. [Note: I tend to ship the Sif/Thor pairing myself, but I already have the comics to enjoy/explore that, and Thor/Jane does work in the movies]
Cut back to the dark elves, and they are ready to begin a war (or at least a strong surprise attack), in contrast to the Asgardian battle weariness just witnessed. Mal still sends Kurse off to pose as a captured prisoner, but references that the stone will keep him hidden from "accursed Asgardian eyes", to explain why Heimdall tries but cannot see/recognize him. The searchers return without having found the aether, but they tell Mal that Asgard is vulnerable, with few powerful allies at this time.
Malekith: Without the dwarves' machinations or the light elves treachery, who will you have to protect you from our wrath?
[Note: this sets up an important weakness to the dark elves: while their ancient age gives them strength (the
Older Is Better trope), it also causes them to overlook the changing power balance in the nine realms]
Cut immediately to Earth and Jane et al exploring the gravitational anomalies [Note: blatantly establish "those pesky humans" as a trope here...even go so far as to have Malekith's "who will you have to protect you from our wrath" spoken directly over top of this introductory Earth scene]. Play the rest of this scene out like the film (but skip the Jane dinner date to start) with the following minor changes: establish Darcy's intern as a suspected Shield infiltrator among the group, as Jane and Darcy discuss keeping the discovery a secret.
Jane: But what about Ian? He'll tell Shield what we're doing here.
Darcy: Come on! Do you really still think he's here to spy on us for them? Jane, the guy just threw our car keys into a space void thingie!
Jane: Well, he's a good actor, I'll give him that.
Also establish that playing with the portals is likely a bad idea, but that it is humanity's innate curiousity that redeems us and why they continue poking at the danger (the addition of the children at this time in the movie was actually a very inspired choice by the writers). Jane becomes too focused on finding the source of the anomalies (instead of just being in the wrong place at the wrong time as in the movie) and becomes the host for the aether as a result.
From this point forward, the movie will play out without too many major changes, as we now move into the much better second act.
- Odin arguing against Thor bringing Jane to Asgard and deriding humanity in general now has new meaning, as the audience knows that Asgard needs the humans as allies, so we side with Thor more strongly in this argument, and anticipate the time when Odin/Asgard will learn what Thor knows: that humanity is worthy of respect and is moving towards being on more equal footing with the "gods"
- show the remaining bits of the LOTR-style opening during Odin's talk with Thor and Jane in the library, spliced together with the invisible ships of the dark elves approaching
- during the attack, Malekith is told that the aether has been detected on Asgard, and he immediately switches his goal to hunting down Jane, and a more ambitious plan
- another change I would make is to give the Asgardian warriors some defense against the laser rifles of the dark elves. Have the guards' shields able to absorb some of the energy of the blasts (at least if positioned correctly), because this is really a "bringing a knife to a gun fight" moment, and it is completely unbelievable that a several thousand year-old enemy is more technologically advanced than Asgard. I would put them on a more equal footing at least, and let the established idea that Asgard is "stretched thin" do the rest in letting the guards be defeated
- after the Frigga funeral and Loki death scene, a Thor/Jane scene is an absolute requirement (I honestly don't know how this was missed in the movie). The Frigga scene would be a "I wish I had known her more, please tell me more about your mother" type of condolence from Jane that would solidify their relationship and could show how much she cares for Thor. The Loki after-death scene would use Jane in the perfect role to represent the audience's conflicted emotions: "He was the cause of so much destruction, but he just saved my life by giving up his own. I don't know how I should feel about him?"
- the cell phone call in the cave from Jane's stood-up date can still work without seeing the actual dinner (or there are other plot devices that can be used to get them out of the cave and back to Earth)
- much later on, change the Ian/Darcy dialogue about "not getting paid to work with Jane" (which is really dumb -- like as in a completely wtf? moment) to a quick admittance by Ian that he was an incompetent Shield employee and was just being transferred around to whichever department would take him, and that dumping him on Jane was just a way for Shield to get rid of him [this will set him up as a sympathetic Woobie character, and his car bit later can work as a crowning moment of awesome, despite its admitted silliness]
- the final battle will change slightly to have Sif and the Warriors Two/Three helping to fight the Dark Elf henchman on Earth. This means less backtracking on how deadly the dark elves are as a threat. More importantly though, Sif et al's presence becomes the big payoff of Asgard finally recognizing the humans as useful allies...in the movie, unfortunately it is only Thor who witnesses the heroic actions of the humans, which gives the audience no payoff whatsoever for humanity's role in saving the nine realms
- also because of this change, Sif can now witness firsthand Jane's bravery in battle, and see her as potentially worthy of Thor's love...have the two talk briefly during the fight, and have Sif acknowledge Jane's existence in some manner
- my last addition, but only if you want to do a "we've come full circle" kinda thing...
Volstagg: Well met, my friends! Asgard owes you a mighty feast in your honour! You fight as though the spirit of Balder the Brave himself was in your blood!
Selvig: Of course! Of course! Glad we could help! [he pats Volstagg awkwardly on the shoulder then turns to confide with his friends] Personally, I'm happier not having gods inside my head anymore!
And that is the end of my changes. I'll stop myself from discussing the reveal of Loki as ruler at the end, despite how it was a "oh for ****'s sake, not that again" moment for me. That is another post entirely.
TLDR version; my changes = more meaningful transitions during the first act, establish Asgard as stretched thin by war, establish Jane et al as pesky humans who will tip the balance against the unsuspecting dark elves and then pay that off with more Asgardians involved in the final battle to witness the deed, fix the after death scenes by following up with Jane/Thor and not cutting too quickly to humourous moments, fix the character of Ian, have a scene with Sif acknowledging Jane, THE END