Executive producer, Hiroyoshi Koiwai, says that her role in the movie will be kept a secret to serve as a wonderful surprise for fans. So far, the idea for her role was made by Keishi Ōtomo, the director himself, but other than that, no news.
Want my first guess? Slap some black hair dye and some asian eyeliner on her and you might actually have Tomoe, Kenshin Himuras deceased wife. That's the only female character I could possibly get excited over that hasn't been cast yet.
In the first film we caught a short glimpse of the back of her head as she cried for her deceased husband. The fact that they didn't show her face still leaves this role very open, especially if more flashbacks are in the picture. My only question though is based on the fact that she's french. I've been trying to do some research on her and I can't seem to find any information on whether she can speak japanese or not, which will be interesting.
So far the only roles I think that are available to her are Shura, Lady Magdalia, Honjō Kamatari, and Tomoe. Of those three Tomoe is the only one that could get me jumping off my seat, not to mention it could deliver a bridge to Enishi thus developing the Jinchū Arc as the story for the last film of the trilogy 'Rurôni Kenshin: The End of a Legend Arc'.
Another catch though is that there's a second cast member who's role hasn't been announced yet either, and she could possibly make a better Tomoe if this be the case. Maya Fukuzawa, I wonder who your role will be as well.
Still much to think about still. Who do you think she's supposed to be?