I don't fluently speak Japanese but I can read hiragana, katakana, basic kanji characters, and I am studying for JLPT N3 in my spare time. I have also played through several Japanese otome games just fine, with some help here and there for kanji words I don't recognize.
Hence, I'm pretty sure Watashi (私) is a more formal / gender neutral version of Boku (僕), which is in the original title, Boku no Hero Academia.
So unless you're saying that Horikoshi himself made a grammatically incorrect title (because all I did was replace the subject "I" with another version of "I"), I don't think I did anything wrong.
But hey, you provide me a solid material that explains why I'm wrong, and let's discuss.