Young Justice, Beyond.

Well, the thought is that, during a mission or whatever. Sportsmaster will be there, and seeing as how he killed Sky (even if he meant to kill Carter) hurt the speedster, he'll do the same to Mei, just to torment him. Like, strike his javalin threw her chest. And as Mei lies wounded just like how Sky did before they died he wonders why she shouldn't be able to heal herself. But Mei explains she can only heal other creatures than herself.
Anyway,they manage to get Mei to the hospital. However, when the doctors are gonna give her blood and asks which blood type she has, they don't know. They take a quick test, only to find out that Mei, doesn't have any human blood type, and is cold-blooded. Meaning they can't give her human blood because they don't know if that's going to kill her.
However, if they don't she's going to die from blood lust. And Wally who can't lose her the way he lost Sky insists they'll do the blood transfusion anyway, even if he knows it's a risk.(Mei is past out this time)
So they do it.
Later Mei regains concience and they all think she's going to be alright.
However after going home and all that Mei starts feeling sick the couple of days after, and even passes out. They go back to the hospital where they realize the blood effected her more, speeding up her "aging" to that of a human and being 116 already she therefore feels so bad.

That's kinda first half of it ^^ what u think. I just got another thought if u wanna involve ur chars though)

((I really like it!! I have a kinda morbid way for my girly to get involved XD But I like it! I do! But will she like age all the way to 116? XD))

(Thanks ^^ Oh, nice. Tell me? Oh. And no. I mean, like she'll start aging like a normal human from 116, so as she looks 20 something... next year she'll be 117, but looking 21, and so on. So, it's gonna notice more fysically compared to if she was still a mermaid, that kinda looks young for so many more years. Are ya with me?)

((Ah, okay, yeah, I got you ^^ And my idea is that Carter finally just snaps; like, her Lazarus Effect finally takes over and she either kills her dad or leaves him hospitalized))

(Oh, bada.ss girl! Maybe that could become an epic fight between her and Sportsmaster right after he hurts Mei?)

((Mmhm! I can see it!))

(Cool!)

((So, shall we begin? Or do you wanna wait for the idea or?))

(Yeah. I think we could. Unless you got something else u wanna do in between)