I'm fine for a few chances to source lore in adaptions, but this one is way too far. If you wanted to say that maybe it wasn't Watson, and maybe someone Sherlock worked with before, then fine. Bit odd, but sure, why not. But these are hallmark characters for British (and to a lesser extent, international) culture. Make Watson female? Sure, that could create an interesting character study and dynamic. Make him younger/older? If you want, same thing goes for this.
But making him an absurdly young cute girl with cute anime hair (who we all know is going to act like a young cute anime girl), and will almost undoubtedly share nothing in common with the character but name and titles, then why even call her Watson?
If she turns out to behave like Watson, or at least resembles him in actions, while bringing in a different edge due to age and gender then fine, I'll happily admit I'm wrong. But I think we all know they didn't make him this character because they wanted to try a new dynamic with the Sherlock character. They did it for marketing and because they wanted a cute anime-esk character in the game.