The goose itself is obviously a very popular character as the main troublemaker. People often write about its mischievous antics in great detail.
The gardener can be a popular character too. Fanfiction could focus on his relationship with his plants and how the goose's interference affects his gardening, as well as his attempts to protect his precious garden from the pesky goose.
The main character is the goose, of course. And there are various villagers like the gardener, the boy, the shopkeeper, etc. who interact with the goose.
The village dog could be a main character too. It could be constantly chasing the goose or getting tricked by the goose into chasing its own tail. It might also have a soft side and end up befriending the goose in an unexpected turn of events.
The lead goose is often quite popular. This is the goose that usually takes the lead in the adventures and is the bravest of the group. Another popular character is the old wise goose that gives advice to the younger ones.
The 'untitled goose game' is about a naughty goose causing mischief in a village. The goose goes around stealing items, honking at villagers, and generally disrupting the peaceful village life.
One interesting plot could be the goose forming an unlikely friendship with another animal in the village. For example, the goose and the cat team up to prank the villagers in a more elaborate way than the goose could do alone.
A crossover with Minecraft would also be cool. The goose could waddle into the blocky world, disrupt the building projects of the players. It might steal their resources like diamonds or iron, and peck at the creepers just for fun.
A simple plot could be the goose getting lost in a new area of the village that has never been explored before. It then has to find its way back while causing chaos along the way.
First, decide which universe you want to crossover with. Then, think about how the goose's personality and actions would fit into that new world. For example, if it's a crossover with a fantasy world, the goose might be seen as a strange magical creature.
The geese themselves would be obvious main characters. Each goose could have different traits, like one being the brave leader, another being the cautious one.
The Brimstone Society members can be popular too. Their role in the game as those who oppose the vampires gives room for fanfiction to expand on their interactions with Rayne. Maybe there are stories about their recruitment of Rayne or their joint missions against greater threats than just vampires.