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Most obviously, the character drawings in this episode are intentionally emulating s1 (more of a vibe thing than an exact "copy" of the style). SpongeBob has his old upturned cheeks from the early episodes. 


Let's start with the most fundamental part of any movie: the story.
Typically when a show does "gimmick-y," really out there episode concepts like time travel or characters going to space, it's a sign that it's been going on too long and the idea well is running dry. However, SpongeBob did both of these in SEASON ONE! 
"Ripped Pants"