User blog comment:Miskos3/Fairy Tail 528: Dragon of Magic, Review/@comment-7418318-20170326143834

Crazy end-times theory time!

So in the MS translation the last page has the text Upon the chaotic battlefield, the "key" to bringing about the light has appeared?! and the next chapter is called "teacher".

So we have someone who might well be Anna from her looks and the mention of both "key", hinting at Celestial Spirit Magic, and teacher. So I guess Lucy will be the pupil and learn something new.

And way back when the series started, Hiro Mashima said that Lucy's name was inspired by the song "Lucy in the sky with diamonds".

Sooo... now comes the crazy theory. Anna will teach Lucy a special technique, one powerful enough to deal even with Acnologia, but at grave cost. It will give her enormous power for a short time, but then transform her into a Celestial Spirit, with all the drawbacks that this comes with, like eternal life and being unable to sustain herself in the mortal world for long.

And of course she'd get her own constellation in the night sky, so it'd be quite literally "Lucy in the sky".