Think about questions that have been left unanswered in your fandom. i.e. What happened during this period of time character A wasn't on screen? What did character B mean when he/she said _____?

Read other people's stories and gain inspiration from that. But please be careful not to copy their story, not even near-copy. Someone did that to me once. When using this strategy, I usually use a very vague idea from their story that probably couldn't even be detected as a link to it.

Think of situations that you could put specific characters through. Combine the situations. Revolve the situation around several characters.

Think of "what if" scenarios and tangents you could use to alter the canon storyline. Sometimes you can get inspired by something random like how someone interpreted a spoiler.