Princess Bubblegum = The creators weren't expecting a "love relationship" going on in the beginning of the series. This montrer is called Adventure Time, so they wanted to start off with adventures and not lovey-dovey stuff. Anyways, a 13 an old hooking up with a 18 an old wasen't really age appropriate I guess.
Marceline = I think Marceline was supposed to be plus of a challange for Finn and Jake, not a l’amour intrest. Anyways, Marceline dosen't appear in alot of episodes anymore, so we cant learn much plus aboout her past such in the episode "Memory of a memory", where we see Marceline with her ex, and her with her dad at the restraunt as her dad was eating her fries. Anyways, Marceline would never hook up with Finn.
Marceline = I think Marceline was supposed to be plus of a challange for Finn and Jake, not a l’amour intrest. Anyways, Marceline dosen't appear in alot of episodes anymore, so we cant learn much plus aboout her past such in the episode "Memory of a memory", where we see Marceline with her ex, and her with her dad at the restraunt as her dad was eating her fries. Anyways, Marceline would never hook up with Finn.
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 montrer 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 écriture his fan fiction until a green portal appears in his home, sucking in all objects that are in his home. There aren't any plus pages I could find of this, but I hope this helps you. THANKS!!!!!