A short and probably non-canon ficlet wrote up on Tumblr analyzing the results of a probable Zukoline relationship after some conversations with fellow Finnceline writer, good buddy of mine and awesome Finn RPer, The Stinky Foot.

...Yeah, I just made a thing for Zukoline. I made it an unhealthy and in-story sick thing, I don't know if this makes me a bad fan or not.

Disclaimer: I continue to not own either Avatar: The Last Airbender or Adventure Time. Crossing them over like this is a pretty sweet way to make up for it, though.

...

Zuko had always thought that 'tryst' was a dirty word and he still thought it was, even as Marceline followed what was increasingly becoming their routine when they happened upon each other in the ship with no one to watch and tackled him into a wall, her vastly more powerful body crushing all possible resistance he might have offered in an instant even though he rarely bothered and crushed her lips onto his, often so forcefully her fangs cut up his lips, and this time that was the case, her knife-sharp teeth moments ahead of her hungry lips and leaving bloody tracks on his mouth just as her mouth sealed on his face, hungry and her arms so tight around his arms that he couldn't move.

It stung for a moment. Blood on his mouth, Marceline's taste on his lips, and she was tasting him, he knew it, her tongue sliding over his lips and painlessly sealing up the cuts she unthinkingly put on him and he didn't notice because she was still kissing him, hands clamped tightly around his shoulders and he half-thought that it was amazing that her coolness was a lovely thing, a coldness pleasant and refreshing and somehow alive, a thought moving through his own arms wrapping around her sides in a desperate hug and squeezing as mightily as he could as he kissed her back with all the intensity of the fires burning in his heart.

He was aware of the metal against his back sliding, Marceline pushing him up and against it so that she didn't have to lean down to kiss him, she was taller enough than him that it was just easier for her to pick him up and slam him against the wall and kiss him so violently it was as though she honestly needed to consume him whole, shut down the desperation and the grief by filling it up with Zuko, and a thought broke through the mindless need between them both when her hand moved from his shoulder to his face, claws restlessly exploring the harsh countours of his scars and he thought again 'She doesn't mind the scars'-

'Neither did Mai,' A treacherous thought whispered back.

Something similar went through Marceline's mind, though Zuko didn't know if it was her suddenly realizing that her lover's hair was coal-black instead of the yellow of young corn, or that his face was too narrow and weathered and burning from the inside. Maybe it was just him suddenly tensing up, and before he could begin to recoil at the wrongness of it all, the unthinkable horror of it ('traitor', he thought of himself, 'treachery to your wife-to-be in absence is still treachery!') and she let him go even before he could push her away in existential self-defense.

Zuko hit the floor on his backside and Marceline took several horrified steps back, her shallow breathing more powerful than usual, lips pulled back over bloodied teeth and her eyes too wide to look entirely sane. There was still a drop of blood on her lips, a testament to their mutual betrayal of their respective loved ones (a dour woman full of disdainful remarks for everyone else and quiet smiles and loving kisses for him, and a younger boy already honorable as a man and loving his friends so much she wanted to keep him forever and ever for the vampire queen).

They breathed heavy for a moment, and then Zuko, his arms wrapped around himself in a tight hug, said, "Marceline…we can't keep doing this."

He looked up at her. She was hugging herself now, and for a moment she loomed over him, a tall and mildly curvaceous figure more threatening than alluring to him (and she scared him even in the best of times; he was the sun, bright and fierce, but she was the night, and he knew that the night beyond the world might one day devour the sun just because it could and then there would be nothing the sun could do to stop the night or save itself), and then she carefully sat down, a gangly and vulnerable woman at least several years older than him and just as scarred on the inside. She whispered something that Zuko didn't catch.

"This is wrong," Zuko insisted.

"I know," Marceline whispered again, loud enough for Zuko to hear her. Her mouth worked around words she didn't dare let leave her throat, face turning more and more turbulent by the moment, and then all she could do was let out a noise no beast could mimic and punch the metal behind her, denting it without even noticing.

Zuko slumped back, hands on his knees, and his throat hitched a few times as he buried his face in his sleeves, wiping away the burningly hot dampness on his face. He glanced for a moment at Marceline, the unscarred side of his two-faced countenance looking at her, and he saw her face buried in her hands, almost bared into fists clenched tight and gray-blue skin turning white from pressure.

They sat there for a while, trying not to cry, keeping their tears and sobs bottled up, memories of Mai and Finn so fiercely close it was all they could do not to scream at the injustice of it, that they were so far from home that lay in those two special people of steel edges behind gloomy velvet and bared metal so bright and shiny you couldn't help but love it in spite of how nakedly blunt it was.

Both of them just wanted to go home. They wanted their loves back, Zuko and his Mai, Marceline and her Finn, and this was just a distraction and a way to drown out their sorrows for a while and feel loved and good and not like the damaged scion of murderers or the monster with the bones of the world still sticking in her teeth, and when they were with each other Zuko could pretend that Marceline was shorter and colder and just totally owned him, and Marceline could pretend that Zuko was younger and smaller and so good it was like owning and loving a sun that didn't burn her.

Marceline clamped a hand on Zuko's knee. "Kiss me," She said, voice flat and low and dangerous.

Zuko's response was immediate, and a repeated statement from before "This is wrong."

"You want this too."

"…I know."

Zuko had hung his hand, and Marceline's hand moved to his chin and sharply jerked his face too look up and see that her face was still inches from Zuko's face, eyes fierce and almost red at the edges from where she'd been crying. "I know too," she said, voice more tired than anything Zuko had ever heard. "I know, damn it. But we need this even more than we hate it, so you listen to me good, buddy-boy, this is sick and wrong and it's like slapping Finn and your Fire Lady every time we sneak into a room where no one's looking and we leave like an hour later knowing just what total jerkfaces we are for stomping all over the real important love there, but I know that you hurt just as bad as I am and it makes the hurting stop just long enough when I have you against a wall or you kiss me out of nowhere or we're both hugging and not in a friend-ish way, and I know we're both sick of seeing practically everyone else on this boat in a relationship or not caring they're not in a relationship, so we can both just deal with it and do this thing and one day we'll find my Finn and your Mai and we'll be home and we'll forget that we ever did this to each other, so just shut up and kiss me, you're just as too much hero as he is."

Zuko's hands gingerly clasped Marceline by the side of her face, palms squishing faintly into her cheeks, and kissed her more softly and tenderly than he had ever kissed anyone besides Mai, and he felt the tears start coming and he let them come even as Marceline hunkered over him and straddled him in a deeply awkward position from their differing heights, kissing him back just as softly, their lips meeting as gently as clouds brushing through one another on a day of smooth winds and her tears mingled with his, dripping past their lips and tasting like sorrow and regret and other things they didn't care about because it hurt so much.

Zuko felt her hugging him as tightly as a child might hug a teddy bear, and he hugged her back, the two of them a pair of lost and lonely scarred veterans of wars that had left them both aching and feeling too old. They couldn't have the boy and the woman that made them feel like good people again, so they made due with each other and it made the hurting stop for a while.

Just for a while. Until Marceline's promise was fulfilled, that was still enough.