Lucy is not a key
Okay so this is my theory on what the heck is going on atm in Fairy Tail…
Let’s go back to what we thought Zeref’s plan was in the beginning: world domination in the current era.
He attacked Ishgar for Fairy Heart, or unlimited magic power to supposedly defeat Aconlogia.
Going back to what we know now, I believe the only reason Mavis is a key to eclipse is because of that power. The gate was originally used to find a time with more magic, right?
Well, like they said in the GMG arc, it takes a whole lot of magic to ‘charge’.
So what if you had unlimited magic?
You could use the gate as you please.
Now, there’s 'one’ more thing you need to open the gate.
The Golden Keys of The Zodiac.
So, like the title suggests, I believe Lucy is not a key, but the keys she holds are.
Lucy and Yukino might be in danger.