I am sure that the universe called Kishimoto's ass can pull a new character.
Ehhhh, he always had logic behind his "asspulls" as people like to call them. Just because you disagreed doesn't mean he made things up, he always had some reason for what he did. People love calling black zetsu's backstory an asspull, I disagree, it neatly wrapped up a lot of loose ends that made no damned sense otherwise. People also like calling the whole Moon People an asspull, but he hinted at it a lot, go back and read the manga, look at all the references between the bijuu's and the moon.
Only way this could be a new character is if
-There was an Uchiha who Itachi trained/loved(who knows at this point) which wasn't in the massacre, which doesn't make much sense since Itachi did the massacring, you wouldn't think he'd forget someone like that(he had to beg the 3rd to save his brother) and even if he did, you think Obito would've let one slip through?
-Orochimaru's experiment (still could be Shisui, Oro may have found him)
-Itachi had a child he didn't know about (wasn't born yet, didn't realize he'd knocked a girl up etc)
