User blog comment:B214/Volume 21 info & spoilers/@comment-68.35.38.210-20160319204302/@comment-77.97.12.117-20160320153836

Most likely the next arc will be about Indra, as the Author himself stateded that he had a plan for that characther earluer anyway. that is most likely the reason that he decided for the whole 'seal-up' of the faction leaders, as  it was sade that it would take the 4 maous at once to check that hod (Without sirzechs and  Ajuka going 'Super Saiyen'.

Most likely the story will be continueing for a good while yet, as it has Issei becoming a high class devil after volume 21 so it will most likely be introducing and building up a good bit of his own peerage and his own experiences as a leader. Possibly with attempts by ajuka to resuce the other leaders from their entombment with Trihexa.

Also,  does anyone else think that the fact that Gabriel is the only one of the 4 great Seraphs to remain signify a greater amount of interact between her and the heroes? Possiby her  gaining feelings for Issei?