User blog comment:IamJakuhoRaikoben/Chapter 394: Juvia vs. Keith, Review./@comment-71.50.113.114-20140725153119/@comment-7418318-20140725161130

As much as I love Lucy's "Aquarius Mode" I think I'd prefer if it's not something she can easily activate at will, but rather something that either acts up by itself or that only become accessible in the most dire circumstance... at least for the time being until she learns to harness this new power.

If the former is the case, it would hint at there still being a connection between Aquarius and Lucy, even though the key was broken and that it's actually Aquarius who uses that power with Lucy acting as a medium, rather than the source of the magic. I can already imagine a scene where Lucy figures this out and is both happy and sad about Aquarius still being with her in a way.

Or maybe have it have some downside, like if she overuses it too much she might turn into a Celestial Spirit herself, leaving her unable to live in the Human World any longer.