joseph probably wouldn’t change at all: impressionable little kid that follows niklas around and attempts to steer him in the right direction but is a little warped himself and finds his antics hilarious, has some deep-seated father issues and a hatred of dominic that his tiny little body can’t contain, good at improvising weapons when his slingshot isn’t enough, morbid sense of humor, takes after his mum in the “screaming and cursing loudly when ticked off” department
although tbh he probably learned all his swearwords from niklas (callie doesn’t swear at all, even when super pissed) so it’d be like [ANGRY GERMAN SCREAMING]
tbh the main difference between canon and au joseph is that he’s also grown up in germany alongside niklas, but he’s still.. english? i wanna say he and callie are english for some reason. don’t ask me where from i don’t know shit. point is he’s a foreigner in germany, even though he was either born there or they moved there a few years after he was born, and as such has a better grasp on pop culture, english/american slang, english in general than niklas does
maybe callie is from england and au!tank is american or something. mr. dejon from fucking texas
tbh he has no idea his mom is psychic and is kind of wigged out that he’s on a grand adventure with a party of them, but probably finds it more exciting than terrifying

dominic is surprisingly well-adjusted, grew up in a big household, as mentioned is on good terms with his biological parents but hasn’t seen them or his half-brothers irl since he was adopted and tbh never met his brothers at all (and would be really taken aback by joseph’s pure unadulterated hatred towards him). still kind of that weird, tall and lanky, socially awkward gay kid with no real skills to his name. probably picked up baseball because of ken, and has a not-quite-fluent-but-close-enough grasp of german to where he and niklas would be able to talk without too much weirdness.
would be really freaked out by literally everything, especially since he’s ken’s son so he probably also has some psi powers that, like in main canon, are really undeveloped and kind of weird. he’d probably feel like a walking pile of trash once he found out why joseph hates him so much but it’s not really his fault so ????
i dont know what his last name would be. what’s a suitably american last name. he probably lives in colorado