So, we are already on the right track with this series with how BMO and his backstory was handled, but my real issue is how they're going to handle Finn and Jake - the FACT that they didn't get effective closure in the original series finale like they should have, after looking at it again (IMHO) has me a bit weary about what's in store for them, honestly.
I almost want to see a show based on Y5, as she has potential to be a great MC in her own right. That way, this series can still continue, while not completely dragging back everyone we know, but instead focusing on new characters like Y5 who can breathe new life into what's here.