Hey guys.  I was looking at stuff on the wiki, found Marceline's Mom, and started doing some thinking.  This is just another crazy Adventure Time theory that I came up with.  You guys can read it, and tell me what you think.  So here it is:

Marcelines mother
In this photo seen in "Return to the Nightosphere", obviously Hunson Abadeer is on the left, then there's Marceline in the center, and possibly Marceline's mom on the right.  To me, what appears to be Marceline's mom on the right, actually appears more like Marceline.  Also, I think it was confirmed that Marceline's mom was a human, right?  So, I actually think the white person in the middle is actually Marceline's mom while Marceline is on the right.  The reason Marceline's mom is shorter than marceline and Hunson is because she's very old.  This would also explain why her skin is so white.  So, if the woman on the right IS Marceline, notice she's apparently closer to her age now then right after the Great Mushroom War.  This would mean Marceline's Mom SURVIVED the War, but grew very old while her family stayed the same because they are immortal.  So, this would mean Marceline's mom died due to old age.

Of course there are flaws, and one would be directly in the photo.  Marceline's skin is naturally pale and white while if Marceline is the woman on the right, her skin is grey like her Dad's.

I hope you liked the theory!