User blog comment:IamJakuhoRaikoben/Chapter 406: The Girl Within the Crystal, Review./@comment-117.205.5.87-20141028094333/@comment-7418318-20141030092915

According to the after words Mashima wrote for the manga volumes, he roughly plans out an entire arc, so no he certainly doesn't write one chapter without thinking of the next.

How or if he plans for certain events in future arcs he didn't comment on as far as I remember, so it's anyone's guess, but just consider that Erza wasn't able to cry from her left eye from the very beginning as a long-term set-up for the Tower of Heaven here. Or how Zeref was already talked about in the very beginning. Or the mystery of the Dragons! Or many other small leads that he gave us well ahead of the arc they were explored in.

Juding by all that my best guess is that he does plan out the key-points of the story very far ahead and Lumen Histoire and Mavis are such key-points.