Thor has the wrong ending. I don’t know that a different ending would have made it a better film, but I do know that a slight change in the final minutes would have certainly made the character arc of Thor himself feel more full, and it would have made the entire film feel more purposeful. How should Thor have ended? Like this:
Thor should have been stranded on Earth at the end.
It’s as simple as that. Everything else should have remained the same, with the final confrontation on the Rainbow Bridge and Thor being forced to destroy the Bifrost, but instead of Loki screaming ‘You’ll never be able to see her again!’ he should have been yelling ‘You’ll never be able to go home again!’ Thor’s decision to destroy the Rainbow Bridge should have cut him off from Asgard.
Leaving Thor in Asgard at the end of the movie represents a return to the status quo from the beginning of the movie, which is always the worst place for a story to go. While Thor has seemingly learned humility (or so we’re told - there’s very little action in the film to truly show this), he’s back at his father’s side, the heir to the throne again. Returning Thor to Asgard is only a satisfying ending if he ascends to the throne, but the mythology of the character won’t allow that. So Thor’s back to being a prince, just one with a better attitude. Loki being exposed and exiled isn’t that big of a change, especially as this film isn’t called Loki; everybody walking into the film knew he would go bad since the character is historically a foe of Thor.
Thor embracing his exile to save the world of the Frost Giants would be a major sacrifice. It also really proves the character has changed - he is willing to give up his home and birthright to save a race of beings he previously considered an enemy. Giving up seeing Jane Foster, as he does at the end of the current version of the film, is meaningless. He’s a thousand years old, and he’s known her for maybe 48 hours. This is the equivalent of you or I having a nice conversation with someone on a short flight; it’s a blip in Thor’s total timeline. Even if the film had successfully sold the relationship - which it simply doesn’t - there’s no way that giving up Jane Foster is that much of a sacrifice. And the film had in no way established that Thor had any other particular love or care for Earth.
But ending the film with Thor trapped on Earth, yet finding some solace in the company of Jane Foster? That’s the beginning of a good romance story. And ending with Thor on Earth gives the film some weight; there’s an arbitrary feeling to this story - why is THIS the first Thor story we’re being told - that disappears if it ends with a huge change to Thor’s personal status.
And those are just the things that would be vastly improved in THIS film; since Thor exists only as an introduction to the character for Marvel’s The Avengers the movie should end in a way that makes him joining the team easier next year. If even a minute of screen time is wasted on getting Thor from Asgard to Earth in Marvel’s The Avengers it’s a minute too much.