Writing an 'always one foot on the ground fanfic' is all about balance. You want to add your own creativity but also stay true to the source material. Start by making a list of the key elements of the original work that you can't change, like the main character's backstory or the overall theme. Then, think about how you can introduce new ideas that fit within these boundaries. For instance, if the original work is about a group of friends in high school, you can introduce a new character who has a believable connection to the existing group. Maybe they are a new student from another town who has a similar interest as one of the main characters. This way, you are adding something new while still keeping one foot on the ground of the original story.
First, really understand the original work. Know the characters, their personalities, and the world they live in. This will help you keep your fanfic grounded. For example, if it's a fanfic of a fantasy novel, study the magic system and how it works in the original. Then, when creating your own plot, make sure it doesn't break the established rules.