You can use phrases like 'years later' or 'after a long time' to indicate a time skip. Also, describe changes in the characters or setting to make the passage of time clear.
Well, one idea could be a fanfic that explores the backstories of the main characters in Don Regular Show and how it shaped their present. Another could be a crossover fanfic where they interact with characters from another popular show.
You might come across fanfic that explores Don's backstory, revealing hidden aspects of his character. Or, there could be alternate universe tales where Don has different powers or a completely different life.
You can do it by clearly marking the time jump and explaining the changes that happened during that period. Make it smooth and not too jarring for the readers.