User blog comment:IamJakuhoRaikoben/Chapter 456: Order, Review./@comment-24039234-20151010154430

Okay, how about this: Mira took over every member from Tartaros whom she could find in one piece (no Keyes, poor guy got Juvia'd to pieces). They agreed to this so she could deliver their souls to Zeref.

My aim is not to make Mira OP (it has to be handled very carefully to avoid that, especially if it includes Mard Geer), rather I'm imagining a great scene, where she "unloads" all the stored souls at Zeref in one swoop. With this, we can have an epic reunion, and maybe use it to make a dent in whatever plans Zeref has, if it fits the story.

If you don't make it too big and keep it subtle, I think this could make a great "kids meet their creator" kinda scene, throw in a hearfelt power of family-speech by Mira how Zeref was an irresponsible "parent", and you're all set. :)