It seems like Mashima is preparing for Aquarius' return, but I'm a little worried about it.
We know that Lucy's spirits are very weak and they don't evolve according to her power level. And in the Elentir arc, Mimi mentions that the spirits are weak because Lucy is weak, she can't maximize the spirits' powers.
Lucy seemed surprised by all of this, showing ignorance (it doesn't make any sense, since Lucy was aware of this at the beginning of the manga) of this information. So there is a possibility that Mashima is preparing Lucy to learn a spell (something like the Star Dress) that maximizes the power of the spirits, making Aquarius' return special.
I thought about this because there is no point in Aquarius returning to Lucy's hands and continue so weak (Mimi easily dealt with Aquarius' magic). It would have no emotion and Aquarius herself is irrelevant, since Lucy has her Star Dress.