The adventures of Sharkboy and Lavagirl.
This movie is my biggest guilty pleasure, my nostalgia for this movie keeping me from hating it despite it's many, many problems. But I feel that with some rewrite this could have been a good movie, or at least a decent one. So these are the changes I would make to make the film stronger. And sorry in advance for any spelling mistakes on the names but I can't be bothered to look up the proper spellings for the names.
Have it not be clear whether Max's dream world and all the characters in it are real: Near the end of the movie we find that the entire time Max was on planet Drool he had actually been asleep, I like this idea because it could have made an ambiguous ending (was it all a dream, or had Max actually transmitted his mind to his made up world?), but then they go and ruin it by having Mr Electric and our two dream heroes arriving on Earth. The ambiguous ending would have left a great sense of mystery and wouldn't have told children that if they dream for it hard enough their fantasies will come true as the movie accidentally ending up teachig children.
Have Sharkboy and Lavagirl based off of the friends Max had at his old school. Again, the movie had something going for it when it had some of Max's dream characters be based off of real people in his real life, so it would make perfect sense if the two biggest characters in Max's dream world were based off of the two people closest to him. Maybe Sharkboy and Lavagirl could have been characters that Max's friends made up and pretended to be when they were younger and did play-pretend, so that's why his friends would appear as they do.
Have the overall villain not be Linus. The movie could still have him as one of the villains but he didn't work as the main villain for me. Going back to the character in the dream world being based off ones from the real world thing, Linus and the other bullies in Max's life should still be bad guys but not the main villain, and when Max learns to defeat them he incorporates that into his real life to get the real bullies to leave him alone or even become his friends.
Don't have Tobor voiced by the same guy who does Mr Electric and Mr Electricidad. I think I understand who Tobor is meant to represent in the movie, he's supposed to be the personification of the ideal teacher; someone who is kind and caring and have a patient and gentle voice. And Tobor has Mr Electricidad's voice because he's the only teacher in Max's life, but this almost ruin's the effect because before Max goes to planet Drool Mr Electricidad hasn't acted like the best of teachers, he certainly wouldn't be my tutor figure, which is why Max imagines him as the big bad Mr Electric. There should have been another teacher-figure in Max's life, either from Max's old school or his current one, that should've voiced Tobor because they were nice to Max.
You're probably wondering who or what I'd have as the big evil instead of Linus. That's easy, what's the biggest threat to a child's imaginary world? I think that's it be reality and growing up. As a child grows up they have to spend less and less time in their imaginary worlds as they take on more responsibility and have to leave behind their simplistic childish worlds in order to do so. That's why reality would be a threat to planet Drool, because it's an unstoppable force that causes a child grow up and leave their imaginary worlds behind, and Max growing up means that planet Drool would be destroyed or at least become dormant or something like that. And Max beats it by accepting it, realising he can't stay forever in his fantasy world and has to grow up and persevere, but that he can still have his fantasy world when he's asleep and dreaming. That way planet Drool continues to 'exist' while Max can still grow up like everyone else.
Have Max's problem be that he doesn't want to grow up and accept reality. This links into what I think the big threat should be. Max's imaginary world should be his escape from his problems in the world, how he copes with arguing parents, bullies and other problems; and Max doesn't want to grow up and face reality because he thinks that means leaving behind his imaginary world and his safe place from all his troubles. But the more he retreats into his imaginary world the more he pushes away the real world and the people in it.
Have Marissa play a bigger part in the movie. This character would have been so forgetful if it wasn't for her defeating Mr Electric at the end, I actually had to look up her name so I didn't have to keep on calling her the Ice princess since that's the name I remember her by. Marissa should be the person who wants to be Max's friend but he pushes her away in favour of his imaginary world, believing she would just turn on him like others have. But Max's imagination creates a character for her in his dream world and she joins him when he get's to planet Drool, but Sharkboy and Lavagirl don't like her because they're afraid she'll convince Max to abandon his dream world in order to grow up. And in the end it's Marissa's character that convinces Max to accept reality but that he can still have his dream world. And once Max accept reality he becomes friends with the real Marissa and they share their dream worlds with each other.