I think that this story could have been done better and if they put more effort into the songs, it really could have been a good film.
Change #1:
No more of this Moana dying/demigod ambiguity. It is foolish, poorly done, and doesn't have any change on this film or her character. So let's just get rid of it.
Change #2:
Maui doesn't lose his powers. It's pointless and only there to serve Moana dying/resurrecting angle that we have removed. Instead of that we get Maui coming up with the idea for her to touch the island under the water, and swims her there with "Shark head". That sounds more like a Maui idea anyways.
Change #3:
Storm god needs to be properly introduced in the opening. He also needs to be a constant threat to the party. Maybe have him sends storms to try to stop Moana from exploring or threatens her island. Make it a need for Moana to go and raise the island to stop him. If we are not seeing him, then they need to be talking about him and worried about him.
Change #4:
Remove bat lady. There is no need for her to be here. She doesn't even tell Moana what to do, just to sings a lame song that gives her an idea. We have switched that to Maui now, so we don't need her. She can show up at the end and be the traitorous henchmen in the sequel, but we don't need her here. Just have her show up and look cool at the end with all the other villains.
Change #5:
Fanboy doesn't freak out Maui, Maui loves being adored by people, so Fanboy should be the person he likes the most on the trip.
Change #6:
We need to bring back Lin Manuel Miranda and get some better music. The first film had some iconic songs. Where you are, How far I'll go, We know the way, You're welcome, Shiny, I am Moana, and Know who you are. All of it was great and memorable. The only thing memorable from this film was when she reprised her line in one single from this film, Beyond, "I am Moana!".
I understand that the music team that was working on the TV series was kept when they changed it into a sequel film, but they really should have got Lin Manuel Miranda back at that point. Moana is about its music as much as its story.