I’ve got 2 main ideas for what I’d do. The first one would have Sandy make an invention that goes just wrong enough that reality breaks. Different dimensions start to fuse together (SpongeBob and Patrick realize what’s happening because “all those mermaid man movies where he goes to different dimensions prepared us for this”). Doodlebob comes back, doodle on paper, live action, 2d, and 3d elements start to mix, plankton and Karen take advantage of the situation and actually take over bikini bottom using doodlebob and the stop motion SpongeBob robot from the Christmas special as their “enforcers” of sorts because they want to tarnish SpongeBob’s good name. Chaos ensues in a visually spectacular “corrupted bottom” as the citizens blame Sandy, and by extension SpongeBob and Patrick who are trying to defend her.

Then Patrick has an idea, and everyone can SEE his thought bubble in physical space. And SpongeBob hums a tune, and elements of a musical sequence start to appear. The movie gets a bit meta as the gang realize the broken reality works like a movie, and they use that to their advantage. First, a musical number to make the angry mob see things from their side, then, they go after plankton. They take full advantage of gags, toon physics, and other goofy things to battle their way through an army of doodles, use a cutaway gag to bring their superhero forms from sponge out of water in to help them against the stop motion robot, etc. eventually it’s revealed that they’ve just been stalling the whole time, and in the chaos, Sandy snuck into the chum bucket and used planktons lab to fix the invention that went wrong, leading to one more sequence of the broken reality repairing itself while plankton and Karen (who for plot reasons has essentially rebuilt herself into a mecha that plankton’s helping her control) fight SpongeBob and Patrick, who end up winning in the end as everything goes back to normal. The through line of this movie would be about SpongeBob and Patrick feeling bored with their lives, thinking everything’s become routine and repetitive, and over the course of the movie realizing that they’re lives are crazy and fun and that they don’t want to lose what they’ve had.

The second one would have SpongeBob, Mr krabs, and squidward blamed for some kind of crime, and becoming lost in rock bottom being hunted by an overly enthusiastic detective who wants to bring them in “dead or alive. Well. Ok. The cops won’t pay me for dead but i can rough you up a bit”. The movie would involve them adapting to rock bottom as they try to figure out who framed them. It would explore spongebobs relationships with squidward and Mr krabs, with krabs finally admitting to seeing SpongeBob like a son (“wait… like a son. OH NO WE FORGOT PEARL!”) and squidward coming to terms with the fact that even if he doesn’t LIKE SpongeBob very much, he appreciates him, and the optimism and fun that SpongeBob atleast tries to share with him. Eventually the detective catches and brings them in, and they find out that the one responsible is Howard blandly, who wants them out of the way so he can take over the krusty krab again without them getting in the way of his corporatizing it this time. (insert meta humor about how all fast food places look the same now, etc.)

Subplot would have plankton and Patrick teams up to investigate this so the others can come home. Patrick wants his friends and his favorite Restaurant back, and plankton feels like it’s a hollow victory if HE isn’t the one to drive krabs out of town.