I love Mia, but I don't think she'd fit in the animated series. It just doesn't feel right and I'm not even that big a fan of Artemis. I don't know... I usually hate when characters that didn't appear in the comics become characters in comic shows, especially if they are big contributors to the show, but Artemis is alright. I think it's weird that they made Artemis Cheshire's kid sister, though. Nonetheless, it's a creative plot twist.
Also, I'd hate if they added Cassandra to the show. I actually like Miss Martian, and unless it's Donna Troy, I don't want any Wonder Girls on YJ. I think Batgirl would make for a good female character on Young Justice. :)
This show has been tweaked A LOT, so I wouldn't be surprised if they added someone to the team that is completely irrelevant (again).