Well, rumor has it that Fionna and Cake will come back, which is probably true. It is seen on the adventure time wiki that theere are adventure time comics for about every show they made. It is shown on one page that ice king is making the sequal to his book Fionna and Cake. The sequal is called Fionna and Cake meet Finn and Jake, which could give us a hint to a upcoming episode. It is shown that ice king is writing his fan fiction until a green portal appears in his home, sucking in all objects that are in his home. There aren't any more pages I could find of this, but I hope this helps you. THANKS!!!!!