Not related in the slightest, but I was wondering something.
If Tartaglia's Foul Legacy form was taken in a modern day AU, what is that? Demonic possession? Werewolf transformation?
Hmm... I guess we could take it in a couple of ways. We could take it in a supernatural direction, where I suppose we could go in quite a few directions, or we could try and translate it into a more realistic human behaviour.
Foul Legacy, in canon, appears to be a physical transformation using both a hydro vision and an electro delusion (visions are bestowed by the heavens, essentially giving regular humans a little bit of god power, delusions are human made copies of the same thing, but they have a horrible effect on the human body, once making someone age incredibly quickly.) It's a fighting style he learned from his master in the Abyss, which in Genshin Impact seems to be a place born entirely of negative emotion. Given that hydro is shown to represent emotion, justice, and adaptability and electro is shown to represent devotion, unpredictability, and eternity... it could be said from that that Foul Legacy is a complete devotion to something like the shadow self, or the shadow of a nation, given that it's a Legacy. You know, when you realise just how horrible your ancestors were or how awful the atrocities your nation has committed are. Therefore, it could be something like abandoning your humanity in order to become a single minded warrior for just one belief or goal, or whatever.
Now that we've established the core of Foul Legacy, that it's the complete devotion to some sort of incredibly strong emotion at the cost of everything else, we can try to translate it!
If you wanted to stick to the more supernatural, but wanted to remain more modern, it could be something like Tartaglia signing away his soul to a demon, probably the avatar of Wrath or Gluttony, allowing him to become a vessel for that demon whenever he wills it (devotion!) A werewolf transformation could sort of work if it was something he got branded into him or something. From what little I know, I think lycanthorpy is usually treated as a hereditary thing? I know it as a metaphor for the cycle of abuse and stuff like that, and I don't think that fully fits, but if it was some sort of devotion to a master or cause that he actively chose, it would work.
If you wanted to go fully modern AU, normal Earth, I suppose... he could work as some sort of police officer or prosecutor, or a higher up on the corporate ladder that is only, and I mean only, concerned with the business and business related matters. I think he genuinely would also work as like a member of the military or something. I guess a bit more of a fun option is an athlete of some kind, or a pop star, anywhere you need to kinda let go of your humanity to be a single minded, single purpose tool.
I do think it's worth mentioning, however, that I think it's implied that he simply does care too much about too many things to fully let go for long. In canon, it seems to do a lot of damage to his body and exhaust it. I think his constant desire to keep fighting is because he thinks that that's his weakness, but I'm no Tartaglia expert...