30,000 year old Ladybug/Marinette but like, for realzies
I have gotten my brain stuck on a really cracky, hilarious AU concept.
Remember that time Mari jokingly said, as Ladybug, that she was thousands of years old?
What if that wasn't a joke? What if there was an Alternate Universe where Marinette was just an immortal stuck at this physical age forever?(to make this AU less weird, lets set that adrien and her are supposed to be at ages 18-20 and not like, 14. Small edit for my comfort+moral compass)
The idea here would be that even Fu did not know who or what he was really giving the miraculous too. Tikki does, possibly the moment they suit up for the first time, but she doesn't really mind? Especially not when she has an extremely experienced near immortal Holder. Tikki can keep this bug forever, if she's lucky and plays her cards right.
Obviously this would lead to Mari being a lot more mature in some ways(only some, she's still biologically locked in the developing brain stage), but she is in fact still an absolute mess around Adrien. Though this time, it's for e very good reason:
Marinette has not loved anyone in a long, long time. Like, thousands of years. Not romantic love, at least. After her age crested over a century she really stopped being attracted to people that were her biological age. Until she met Adrien. And she LIKES Adrien. He makes her face flush and her hands get clammy like she's 45 years old again. But Adrien is also only 18-20. He's a baby compared to Mari, a fraction of a fraction of her age. is it ok to even have feelings for someone when even the oldest humans on earth would be still far outside your age range?
Marinette doesn't know. And it's that extreme moral and emotional conundrum, combined with feelings she hasn't felt in a long, long time, that lead to her being a babbling mess around him. Her brain gets locked in self argument mode every time they do anything that could be even vaguely more than platonic.
All of Marinettes weirder tendencies and manic actions? The result of an immortal who, despite living through them, still struggles to adapt to the massive cultural shifts she's lived through, and sometimes still fucks up modern rules and sensibilities.
Marinette would be a lot more patient in this though, she's already a brilliant planner in cannon. So a Marinette with infinite time would have massive sprawling plans that take years to reach their zenith, and they'd mostly all work.
Also really funny things that come from Marinette being immortal:
Chloe is not actually a threat or a real bully to her. Instead, Marinette actually loves Chloe. She never had the chance to get the modern highschool/college bully experience when she was originally a teenager, because one, schools likely weren't a thing, and two, girls wouldn't have been allowed to go if they were.
So Marinette is absolutely fascinated by Chloe Bouregiouse. And she also thinks Chloe is hilarious. She struggles very hard not to burst out laughing as she plays shy, clumsy Marinette(the clumsy is not an act, thousands of years and she's still at least a little bit of a spastic mess on occassion). Chloe is her favorite entertainment. Every over-engineered insult, every petty mean girl sitcom thing, they are like the funniest part of Marinettes larp as a "regular girl TM". She also worries about Chloe though, and is, through careful manuvering, trying to slowly guide Chloe away from her shitty parents and into a better environment, better rolemodels, and a better life. It's taken some doing, but she's getting their. And honestly, the bee miraculous "accidentally" falling into her hands definitely helps speed up, especially since she hero worships ladybug, which Marinette also thinks is hilarious.(she was quite proud of herself for thinking of that queen bee scheme, she even had Fu thinking it was an accident. She of course, shared everything with Chat.).
If Chloe is funny though, Lila is her favorite mental dueling partner. It's actually kind of adorable to Marinette, because Lila is actually this young, and she thinks she's so clever. But Marinette saw through her day one. Like, almost all the way through her. Lila Rossi's manipulations are amateur at best in Mari's eyes, they can't even throw a true wrench in her 6 month plans, let alone the years long ones she's been putting together.(to explain how Mari has known the class longe enough to pull these things off: 18 year old Marinette is still pretty short and small, and can easily pass as 15-16 year old Marinette, so she did got to school with them, just not as long as she did in cannon because you know, no baby Mari to do school.)
The Akuma's are new though, and that's exciting. Mari has seen a lot of weird magic in her time, but she's never directly encountered one of the fabled Miraculous. She's heard of them, and even been on the periferee of conflicts or events that involved them, but she's never directly faced one down before. She's also never wielded one before. She expected Tikki's power to feel overwhelming, tempting to misuse, like the magics she's found before and discarded because she doesn't trust herself with them. But instead, the Ladybug miraculous has always felt like an extension of her own being. She has never once been tempted to miss use this power.
Also, having Tikki, a fellow immortal, around is also really, really nice. They have Immortal Grls night a lot. It's great.