Another interesting idea is a time - travel fanfiction. They get sent back in time and have to deal with past versions of themselves and the guild. Natsu has to learn from his past mistakes, Mirajane has to come to terms with her past self's insecurities, and Erza has to use her past experiences to guide them. Their interactions and how they grow as characters in this scenario can make for a great fanfiction.
One idea could be a story where they go on a quest together. Maybe they have to find a rare magical item. Natsu's impulsiveness, Mirajane's charm, and Erza's strict leadership combine in interesting ways during the adventure.