User blog comment:Cookievalenzuela666/which girl is going to get their own volume next?/@comment-25092626-20140709074557/@comment-3965848-20140709103841

And I think Xenovia might be vol. 19. I can only assume that after that will be Kuroka or Le Fay (her intrests in Issei seem be growing as she also wanted to go on a date with him). If not then i think it might be a guy insted, maybe Vali, Saji or anyone else.