User blog comment:Miskos3/Fairy Tail Episode 258: Tartaros arc: The Fire Dragon's Iron Fist, Review/@comment-5894890-20151107042737/@comment-7418318-20151107135111

What else could be filler on a purely theoretical basis?

Zero, Ice Trail, Blue Mistral, Fairy Girls, Light Novels 1 and 3, possibly even the rumoured Gajeel spinn-off, the first draft plot of the KotSS arc (the one with Lucy dying and Natsu going to the afterlife to save her)... and of course a whole lot of stupid or not stupid filler stuff.

For example they might revisit the first KotSS plot that Mashima rejected with Lucy dying and Natsu going to the afterlife to save her. They might how the travels, training and jobs of the characters between the Avatar and Tartaros arcs. Or they might come up with any number of original stories that have little to nothing to do with the manga story.