Friendship is a common theme. Since they are from different worlds (Marshall Lee from Adventure Time and Gumball from his own show), exploring how they become friends is often in the fanfics. Another theme is adventure. They usually go on some sort of journey together.
One common theme is the contrast between their personalities. Marshall Lee is a vampire and has a bit of a dark, cool side, while Gumball is more upbeat and innocent. Fanfics often play with this contrast. Also, discovery is a theme. They might discover new places or powers in the fanfics. For instance, they could find a hidden realm in the Adventure Time world that only they can access because of their unique bond.
The themes of acceptance and mutual understanding are quite prevalent. Given their differences, fanfics show how they come to accept each other's quirks. There's also the theme of facing challenges together. Whether it's a magical curse or a powerful enemy, they work as a team. In some fanfics, there's also the theme of cultural exchange, as Gumball brings his own set of values and behaviors from his world, and Marshall Lee does the same from Adventure Time.
Chapter 476, "Make a feint to the east but attack in the west, take something out of a bag effortlessly", is the clearest early answer because Make gets a payoff that changes the stakes.
Chapter 444, "Wait and See", is the clearest early answer because it turns Sister Li's situation into a concrete plot event. Early on, Carrying a backpack, Yu Li walked for over ten minutes but ultimately.... By the end, [HAHAHAHA, they're out of luck running into Sister Li!] [Jiang Li is....
Chapter 505, ""Postpartum Care for Sows", is the clearest early answer because it turns Postpartum Care's situation into a concrete plot event. Early on, Zhao Lang laughed, but his eyes became considerably clearer, "Is....
Chapter 378, "What if I said I have a way to find your daughter?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter gives readers a real answer by opening on He Kejun's disappearance was related to an apprentice she once...
Chapter 391, "Has another top celebrity had a scandal?", is the clearest early answer because it turns Destiny's situation into a concrete plot event. A useful checkpoint is the way the scene starts with You were born in a rural area, not exactly poor, but not particularly...
Chapter 578, "Eight Men Sit in Your Lap and You Don't Flinch at All?", is the clearest early answer because it turns Ai Xiaochun's situation into a concrete plot event. Early on, This is ten thousand points we're talking about!" "Didn't you read....
Chapter 433, "The Young Master Has His Own Ideas", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. It opens with Since Jiang Yan and his group left, the house had become much colder.
Chapter 617, "Moral Kidnapping? No Such Thing!", is the clearest early answer because it turns Moral Kidnapping's situation into a concrete plot event.
Chapter 670, "Sister Will Support You", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Li, sensing that something might have gone wrong, quickly used...
Chapter 581, "Where is everyone? Where have they all gone?!", is the clearest early answer because it turns Sister Manman's situation into a concrete plot event.