It's about Kirby's adventure in the Forgotten Land. Kirby explores this strange new land, encounters various new enemies and challenges, and uses his unique abilities to overcome difficulties.
Well, the King Dedede weight gain story could be that he started eating a lot more of his favorite foods like big, juicy fruits and large cakes. Maybe he became a bit too fond of the luxurious feasts in his kingdom. As a result, he gradually put on weight. He might have noticed his clothes getting tighter and his movements becoming a bit slower due to the extra pounds.
One common theme is friendship. Female Dedede is often shown building strong bonds with other characters in the fanfic. For example, she might team up with a group of misfits to take on a powerful enemy.