In the episode 'Take Her Back', Marceline had two dreams of in which both of them she has grown 'old and withered'. The first dream of her visiting Betty and Simon while they bake a pie and Marceline appears older, as if she has biologically grown. Also in her second dream, during her 'poison coma' she has clearly aged significantly and has trouble hearing. These dreams may be foreshadowing an upcoming death for Marceline. This is also further proved at the beginning of the poison coma dream where Peppermint Butler is seen digging a grave, portraying his knowledge of Marceline's impending death. Death may come to Marceline through her somehow being able to age normally and she would pass of old age, as suggested through her dreams. It can also be perceived as her dreaming of actual future events, as a way to see her 'life flash before her own eyes', as in reality Marceline was dying of the poison.
This is what I want to happen as well as a prediction. If Marceline does come to a stage of her actually dying, not by old age but maybe in combat or losing her powers etc, this will probably occur at the show's finale. Marceline would somehow end up in Princess Bubblegum's arms, maybe in the rain for a cliched dramatic effect, and Bubblegum is in harsh tears. She is desperate to keep Marcy alive but knows no avail. It is hopeless.
"Bonnibel. Hey." A dying Marceline smiles weakly up at Bonnibel, both knowing that this is their last moment together.
Bonnie solemnly cradles Marceline in her arms, "Oh my Glob, Marcy. I am so sorry it had to be like this."
"Hey now, Bonnie," Marceline consoles, reaching her hand up to brush the soft gum hair out of the Princess's face. "There's nothing for you to be sorry about. Besides, I have always wanted to die in the arms of a princess."
Bubblegum makes an effort to sadly smirk at the smart-ass comment. The remark itself isn't funny, more sombre even. It is just so typical for Marceline to shed some light on such a dark situation like this one, even if it is with humour. Cold raindrops crash onto Marceline's delicate face.
"To be honest Bonnie, this is the only place I wanna be at right now - or ever. Right here with you."
Bubblegum's eyes has grown wide. She guesses she had known it all along but that confession still sends massive shockwaves down her spine.
Bonnibel looks straight into Marceline's wavering gaze, whilst interlacing her pink fingers into Marceline's. The hot tears continue to stream down the Princess's soft cheeks.
"Whatever happens from now on, know that I've always been and always will give you my everpresent love."
Marceline nods her head in satisfaction, finally hearing the soothing words she had only dreamt of her entire life. Using her last bursts of energy, Marceline pulls her face closer to Bubblegum's without breaking the tense eye contact. Marcy's eyes finds their way shut as her shivering lips meet her fated loved one's pink lips. Their wild tongues unexpectedly increases in speed, swirling and exploring each other's with passion, as their love radiately shines throughout the vast lands of Ooo.
Marceline pulls away to look into her friend's eyes, filled with sadness and gratitude. Marceline watches the detail in Bubblegum's eyes as they dance around and glitter like portals to another galaxy. The princess observes the vampire's wonder-filled eyes, as they twinkle with awe and amazement. But ever so slowly, her pupils roll to the back of her head as her eyelids fall heavily. In an instant Marceline flys away, but she leaves unresentful and content.
Bubblegum is still there with what is left of her best friend in the jarring rain. She hangs her head low and grasps onto the body more tighter than ever, knowing that one day they will once again be reunited, and be free to be whatever they wanted to be.
(prediction to fanfiction, 0 to 100 real quick, got carried away oops. Marcy probably reappears in the nightosphere in a ghostly form)
After reading this again yes, I know that this specifically will not actually happen, but death for Marcy might. D: