Here are some ideas for season 4/5. This may air in 2018.
Anais looks for a Valentine
Gumball gets his mom's strength and the rest tries to fix it
Nicole runs into a potion that makes her rainbow coloured and her family tries to bring her back to normal.
Gumball, Darwin, Anais, and Richard (even though he one) meet Nicole's parents and tries to scare them away.
Gumball finds a toy train that is cursed.
Gumball and Darwin find a monster in the closet.
This is a Halloween special and a spoof of Resident Evil.
Darwin destroys the robinson's house by accident and becomes evil.
Gumball must pass a big test in order to switch from public to private school
Darwin finds a strange cave, and gets lost, so Gumball must find him.
Gumball gets his sweater torn and must get a new one. But, they are sold out of sweaters, so Nicole buys Gumball a jacket instead which makes him more cooler than before.
Darwin gets bit by a vampire and tries to bite Anais and Gumball.
Anais lies that Gumball was a famous person, gets a little overboard and the lying becomes reality.
Gumball and Gummy are on a faceoff to be class president.
Anais tries to find her hidden talent.
The Trap: Part 1
The Whirlettes and Skydancer get traped in a tar pit, Gumball and the gang try to save them but Darling ends up drowning in the tar.
The Trap: Part 2
Darling is still drowning in the tar and Darwin (sort of) tries to save her from the tar pit.
The (Evil) Cake
Anais's birthday cake makes her evil and disloyal to her family.
Masami tries to help Gumball and Darwin to control their anger.
(The) Earth Day
Darwin doesn't know how everyone can make the world a better place.
Gumball and Darwin prank Dad for a sausage and must get him back before they get into trouble.
Gumball Destroys Banana Joe's Cousin
Gumball and Darwin want to retrieve the ultimate Ripleybox controller
The Wattersons' buy a new house
Gumball steals Tobias's snowmobile
Nicole gets arrested when she gets pulled over and doesn't have her driver's license
Gumball causes the school to lose a football game when he keeps getting the team penalties.
Gumball and the gang want to visit a new playground.
Gumball bets Tobias and the gang 100 dollars on a race. If he wins, he keeps his money and is "Too Cool For School", if he loses, Tobias takes his money and makes him do his homework for a month. So Gumball has to prepare fot the big race
Darwin takes an embarrasing selfie and needs to take it down.