After reading through this thread, I couldn't restrain myself. So I joined.
I'm not entirely sure who I'd like to be the LGK. There are several things that I know I do want to see in the actress, and a few things and people I wouldn't be able to bear seeing as the LGK.
Firstly- this is not necessary, just my silly idea- I think the LGK should have darker hair. I think a red head might hurt the eyes a little in a poison green gown. Black hair might be overpowering, remembering that this woman has spent about ten years, maybe more, living under the earth, only returning to the surface occasionally.
This leads me to number 2- she must be pale. A northern witch, living under the ground for a decade would have to be.
Thirdly- I don't think that the LGK and Jadis are the same person. Therefore, I think it would be a very big mistake to have her played by Tilda. IDC what you think, this is just my opinion.
Fourth- she needs some kind of accent, or something. American or British or European in general... It needs to be soft.
Fifth- the super "sexy" actresses are all fine, but I don't think they'd be appropriate for this role. LGK is seductive, sure, but remembering that 1) they are bound by contract to make this movie PG, and 2) Lewis wrote the CoN as a children's series, LGK cannot be "sexy". If anything, mere hints would work. Subtlety seems to be her strong point, so that's how I've always pictured her. Sweet, Innocent, yet dangerous. There's something dark lurking beneath the surface.
Sixth- I've always pictured her in her mid to late twenties or early thirties, or appearing to be. She appears young as a matter of choice. I mean, who'd want to look like a hag? No one willingly.
See, I can't think of a single actress who would do well at LGK off the top of my head, but Emily Blunt has been the best suggestion that I've seen. Maybe there is someone else that I just don't know of. -shrugging- Alright. Wow. I didn't intend for this to be so long.