I'm hoping it's the extra chapter where Natsu reads a book on Lucy's bookcase but burns it to a crisp instead and tries to hide it the fact from her by "borrowing it". Lucy though keeps gleefully asking him how he likes it since it's very important to her since it was a memento of her mom. Natsu and Happy freak out about destoying such an import item to her and they end up scouring though every bookstore (even in other towns) trying to find her another copy... and that's not even the best part (which I won't post due to spoiler reasons)
But it's chapter 195.5 so it falls directly between Edolas arc and S-Class trail arc
I doubt this one will become an OVA though, for reasons I can't spoil