User blog comment:Omega natsu2/Chapter 371: Tartarus Arc, Part 2: Song Of The Heavenly Dragon, Predictions./@comment-198.142.230.136-20140210052414/@comment-7418318-20140212004135

So rather than Lucy being the Celestial Dragon (Slayer) she summons the Celestial Dragon, as in a Celestial Spirit somewhat reminiscent of a Dragon. That makes a lot more sense than an actual Celestial Dragon or Celestial Dragon Slayer.

But I still think it's about Wendy or Grandeeney, since in the chapter title there's tenryu, not seireiryu.