Here's some ideas for season 6.
In order to see the movie, Gumball and Darwin must stay up one whole day to see the movie marathon.
Gumball gets into a battle with Banana Joe.
Gumball tries to be like a dinosaur, but endsup making a fool of himself.
Gumball and Darwin get stuck in pipes and trie to get out.
Gumball finds this weird mansion that is haunted.
Gumball saves Pennys life but must stay with him as favor.
Gumball and Darwin get blackmailed by a stranger and get told to get a job.
Gumball finds about Penny's secret.
The Amazing World of Regular Show 2: When 2 Heroes Unite!
Mordecai, Rigby, Gumball, and Darwin must go to prom night and get into a fight, and must defeat Bobert, and Peeps to save Anais.
The Amazing World of Regular Show 3: The Ultimate Enemy:
Gumball and Darwin get Fs and meet Mordecai and Rigby to help, while they make an enemy that becomes too powerful to defeat.