I wish they had done more with the "Raya doesn't trust Sisu" subplot than they did because all of the pieces were right there and they didn't do much with it until the very end.
Like. Raya hasn't trusted anyone and has been searching for Sisu all this time, thinking she can fix everything. And she finds out that Sisu is nothing like Raya thought she would be - she doesn't seem to take anything seriously, she doesn't have any useful powers of her own, she isn't "the best at being a dragon", and she isn't even the one who created the gemstone in the first place.
So Raya thinks this whole journey and getting the gem pieces is going to be all on her, while Sisu is just going to be the one to put the final pieces together - again. So Raya doesn't trust Sisu enough to let her do anything on her own or trust Sisu's judgement of other people. Like canon.
But this time, Sisu is completely aware that Raya doesn't trust her, and it's not just that Raya doesn't trust other people, but also that Raya doesn't trust her or her abilities. Sisu already has obvious self-confidence issues and how Raya views her would effect her. So by trying to get Raya to trust other people, Sisu's also trying to get Raya to trust her.
Here is my idea for how the story could have changed to have Raya come to trust Sisu and by extension, other people:
Have Sisu be proven right in Talon. Raya still doesn't let Sisu go into the city Sisu still goes into the city anyway, Sisu still gets in trouble with the whole "credit" thing. But when Dang Hu (the old woman and new chief) comes in, Dang Hu doesn't backstab Sisu. She brings Sisu to the chief's house and asks her directly about the gemstones. Not to steal them, but to ask why she wants them. Raya is there (and Noi and the ongis), hiding and listening in, waiting for the right moment to burst in and save Sisu from an obvious trap.
Instead, Dang Hu and Sisu talk. Dang Hu asks Sisu why she wants the gem piece. Sisu explains, in her awkward way, trying not to give away too much while also doing exactly that, but her final answer is that she and her friends are trying to save the world. She's earnest and honest in her explanation, expressing her belief in the people of the world and that she knows they are fundamentally good because that's why she and her siblings were here in the first place (accidentally exposing her dragon truth but covering it up right after). Dang Hu does not respond right away, and Raya prepares to attack, but then Dang Hu talks about her son, Dang Hai. How she had been wary of letting him go to Heart six years ago but trusted him to take care of himself and their people. And he did, because he took a gem piece and brought it back, using it to protect their people. He was turned into a statue by an accident on the mainland, protecting someone else and tossing the gem piece to them in his last moment, asking them to bring it to his mother.
While Dang Hu doesn't want to give up the gem piece, and she still doesn't have faith in the other nations, she still cherishes that the person Dang Hai saved had brought the gem piece to her instead of keeping it for themself. So she decided to pass on that trust to Sisu, giving her the gem piece and asking her to save the world, and bring her son back to her. Also hinting that she knows Sisu is a dragon because Sisu failed to cover it up, and also because Sisu is making fog and that's suspicious enough on its own. Sisu gives the chief the presents she got for her.
Sisu leaves unharmed, with a gem piece, and meets Raya and the baby/ongi squad outside. Raya heard the whole conversation and is obviously shaken by this, but Sisu is all "I told you so!" and very excited about the whole thing. Raya smiles and they all head back to the boat together.
So when they get to Spine, Raya is still hesitant, but Sisu is sure that this time will be like the last time. It goes like it goes in canon, with Tong capturing them, but once he joins them, it still shakes Raya's "cannot trust anyone" belief because that's now two times Sisu has ultimately been right about trusting someone.
When Sisu brings Raya to her siblings statues and reveals why trusting others is so important to her, that, and hearing about Sisu's more powerful siblings trusting her, as well as seeing Sisu using her new powers, helps Raya with finally coming around to trust Sisu, even if she doesn't totally trust Namaari.
Unfortunately it still goes like it does in canon, with Raya's trust in Sisu losing to her distrust of Namaari. Sisu gets shot, Sisu dies, and the Druun take over. Raya goes to fight Namaari.
But this time, when Namaari says Raya didn't trust Sisu, it hurts because Namaari is right. Raya didn't trust her, not until the last second. And not even then. And Raya knows that Sisu knew Raya didn't trust her, and that hurts even more.
But once again things go like in canon and everything turns out okay in the end, with everyone getting their loved ones back.
I'm rambling a lot so maybe this could be worded better but there was a lot of potential in this movie and it didn't feel like it lived up to everything, so here's a way to fix at least one thing.