I just saw an interview video featuring Jeremy Shada and Olivia Olson. And the latter wants to see Marceline's mother. At least that audience doesn't have to ask about Marcy's Mom. Now that we're talking about it, I'm beginning to think that Marceline's bass once belonged to her mom, that's why she's so obsessed with it. Watch It Came From The Nightosphere to see what I mean.
Maybe the upcoming mini series will delve into that. It is said that PB unleashed "ghosts from Marceline's past". So we'll see Marcy's mom, and possibly the vampire that bit her.