User blog comment:Bato of the Heavy Iron Rock Hammer/Will mentioned characters make an appearance?/@comment-25213703-20140723014854

I would say it is highly unlikely they would receive a chance to appear, but still possible so long as Hiro Mashima has space for further future expansion on the plot. Presumably they would could become future arc characters who will not have any significant value in the entire story line of the series, except maybe the arc they would be in.