User blog comment:IamJakuhoRaikoben/Chapter 352: The Flame's Voice, Review./@comment-16214733-20130922174024/@comment-7418318-20130923085916

I doubt the Kotss characters will ever find their way into the manga. If Mashima did that, he'd also have to write a lot of "Do you remember when..." stuff so people who read only the manga also know who those people are.

And if he beings back Oracion Seis, they'll probably have a large enough gap in their backstory so that Kotss isn't directly contradicted, but won't be referenced either I think.