Hm, vampire ideas just reminded me of Code: Vein.
You can kinda slot anyone in there and even stick the setting inside most others, whether as an ISOT, part of the future, or that Thing Over There That Just Woke Up.
Of course, with the way the memory stuff works, it might be better to start the story at the Cathedral, where the MC regains access to their lost memories as a Revenant Soldier. From that point, a Revenant girl named Tanya becomes Tanya Degurechaff, and remembers ALL of it!
You could keep it in the Youjo Senki world if you wanted, either as an ISOT or apocalyptic future, too. Just have Computation Orbs be lost tech, at least for those in the Gaol.
Not sure what kind of story to make there beyond, "Tanya is now the hero in a post-apocalyptic world besieged by endlessly re-spawning monsters having to deal with how her amnesia-driven self has basically turned her into the Beacon of Hope for humanity's immortal vampiric defenders." But I'm sure there's something someone can work with.
On a totally unrelated note, I had a random idea about what Tanya would do if she had a hanger-on in the form of a twin sister. Of course, something like that could push everything off the rails, so I imagined it with the caveat that Tanya has gone through most of canon and then died for some reason. Instead of 3rd Life #84756, she wakes back up in the orphanage as baby Tanya, starting ALL over again. Only this time...
Enter, Tyana, Tanya's oddly positive twin. (Ti"tania" and Ta"tiana", variations of the name of the Fairy Queen) Tanya's suspicious of her, at first, wondering where the heck she came from and why she showed up this time but wasn't there on her last go-round. Eventually, though, she accepts that this is not a ploy by Being X and accepts her bubbly sister for who she is, weird timeline anomaly or not. However, Tanya thinks she's ready for what the world has to throw at her, now. She knows what not to do, and to warn the General Staff ahead of time about getting Dunkirked by De Lugo. The only problem is Tyana is not having it, and makes sure they don't get tested, among other things. Wondering what made her sister decide to sabotage the test and why she has passports, Tanya demands an explanation, but all Tyana will tell her is that something really, really bad is going to happen soon, and they need to leave.
Turns out, Tyana is also reincarnated. Tiny blonde orphan Magical Girl in the Not!German Empire is just a favored template, apparently, and somebody double-booked this one. However, this person is not about to play this ball game. They read their history, and they know that they are about to hit the World Wars, with just enough time for them to reach adulthood as the sequel starts up, and they are obviously Slavs going by their surname, at worst, or just the right combination of phenotypes for some bureaucrats in the Volksaufartung divisions to decide they need a quick name-change and trip to the breeding camps. And if they had tested positive for magic, that might have made things worse! It would have certainly made escaping harder since the current government would want to recruit them if they did. Having to fight in the misnamed War To End All Wars was bad enough, but surviving just to end up forced to make magic Aryan babies is a nightmare from Hell.
She doesn't tell Tanya all of this, though. She dresses it up in fears and ideas that a kid would understand. After all, her sister is obviously a weird but perfectly normal kid given the circumstances. She gets that she might be a bit patriotic thanks to only ever hearing church sermons and political propaganda all their lives, but that'll be fixed once they get out of here and on a boat to the States. Sure, it's not the best time period, but women just got the right to vote, drinking should be relegalized before too long, and they'll hit the job market right in time for the Great Depression to end. It's practically foolproof! And since they're twins, they can stick together and watch each other's backs even if no one else will. And if Tanya decides to get married at some point (to a very carefully vetted man, as Tyana will not let her sister marry any potential wifebeaters), Tyana will be right there to support her, living in one of the smaller rooms and bringing in a little extra income to support her existence as an eternal third wheel. They can live to be old and happy and not get turned into Nazi baby factories or stuck in gas chambers for being the wrong kind of white.
On the other side, Tanya is worried that her obviously delusional and possibly paranoid sister is going to get them killed, maybe in front of a firing squad, since she's pretty sure that's the maximum punishment for dodging conscription. Tyana obviously thinks they have a better chance in America because of all that 'Land of Opportunity' propaganda that came out of there and that one Valleywood movie they watched, but she also can't just turn her over because this is her sister and she can't restart her military career with the reputation of having thrown her twin to the courts and watching her get jailed or die. Nobody would ever trust her with anything! She might even face a case of friendly fire this time!
Somehow, neither of their plans work out, and the two end up in Not!Brazil. They aren't sure why.
~
Silly idea throughout, I know, and I got stun locked trying to find out what the conscription laws for Imperial Germany were but only findingthe Nazi ones. Then I saw the Luce and Tanya pic and fell into a hole
I keep imagining this going like a Looney Toons scenario. Finger in gun barrel. Tries to shoot. Chamber pops like a firecracker. Tanya has a soot-covered face.
Because in Japan, anime just means cartoons. Looney Toons is an anime.
And she's got the power of God and Anime on her side.
