When will she reveal herself? As in show her face!
Mystery No More
In Chapter 312, in the last part in the manga, it's revealed to be Future Lucy asking Natsu to lend her his Fire Dragon Slayer magic, which shocks the whole group (except Arcadios who is still unconscious).
Hint: In Chapter 292, when Datong was looking at the portrait of Acnologia battling a hooded person, it was Future Lucy with Natsu's Fire Dragon Slayer magic!