Hashirama might be the one who's sealing Madara
As we've seen in the latest chapter, the first Hokage Hashirama has been brought back (again) through Edo Tensei. Suddenly I remembered the chapters where Kushina is talking to Naruto about various things.
Among those topics were also the relationship between the Senju and the Uzumaki clan, and the sealing techniques of the Uzumaki clan.
I think Hashirama might have learned a / some very cool sealing technique/s in Uzushiogakure (Eddy Village), and as Kishi likes foreshadowing things, this could be one of those hints he's always giving. Also, it was often said that Hashirama is the only one who could beat / stop Madara.
But maybe I'm just reading too much into those "hints"