I just thought about it after watching a few clips with Neil Gaiman on the BBC Doctor Who webpage. They showed the scene where Idris says something along the lines of "You're like a boy trying to build a motorbike in his bedroom". I swear, it might be a similar accent issue, but she did sound just like Helena at that moment. And it blew my mind. Helena would make a PERFECT Idris. Plus, I bet she would never refuse to wear a victorian dress and ramble quickly at the Doctor's face. Aww, my dreams and I...