I don't like that idea, personally, even as someone with a non-traditional religious background — there's just something a bit too weird and even slightly creepy about it, for me at least. Aslan is meant to be unsettling, for sure, but in the sense of overwhelming awe, not uncanniness. And it could too easily end up coming across as ridiculous, with a lady's voice coming from an outwardly masculine creature. Aslan’s voice is supposed to be deep and rich and wild, and I honestly don't think Meryl Streep (or most other female voice actors) could manage to create the right impression. (I think I've said before, I love reading the Narnia books and other classics aloud, even just to myself for fun, and as a woman, I simply cannot go anywhere near deep enough with my voice to make Aslan sound the way I imagine he should.)

And also, what third-person pronouns are the other characters going to use when referring to Aslan, if Aslan is genderless? 

Wrong will be right, when Aslan comes in sight. 

At the sound of their roar, sorrows will be no more. 

When they bare their teeth, winter meets its death. 

And when they shake their mane, we shall have spring again. 

... Nah, doesn't quite work. {ym}:eyebrow:  

I actually thought of Meryl Streep as possibly portraying Aunt Letty. Though Mrs. LeFay does sound like an interesting choice. So maybe in the scene where Uncle Andrew tells Digory how he came to make the magic rings, a flashback of Mrs. Lefay while he is telling it in V.O. So that would be an interesting idea. 

Also, there's a certain balance in having both a male villain and a female villain (admittedly much stronger and scarier) who both, in different degrees, indulge in pride and arrogance and the desire to control and use others. It's good to show those failings (outright sins, really) aren't confined solely to either men or women. Having two female villains — Jadis and Aunt, er, Andrea — would diminish that strength of the story. Ironically, in the face of all the allegations that C.S. Lewis was sexist, it would potentially make the story sexist where it wasn't originally (because now both the main baddies would be women). {ym}:eyebrow:  

If Meryl Streep really is taking a role in this film — and like others, I'm a little more assured now that it's not Aslan, but I'm not completely confident there — I would also say she'd be ideal for either Aunt Letty or a flashback to Mrs Lefay.

Nor can she play Andrew  Ketterley, who fancies this amazing woman (Jadis)  and begins to think she might fall for him. 

Aunt Letty is much more likely. 

I wonder, what if Meryl Streep is up for the role of Uncle Andrew in the movie instead of Aslan?  The character would probably be renamed "Aunt Andrew" or something similar. Or, Aunt Letty could be given "Uncle Andrew's" role from the novel.  The movie could still be called the "Magician's Nephew" if Meryl Streep played Digory's michevious and devious aunt.  TBH, it would actually be a more meaty role for her acting talents than Aslan (who tends to be portrayed a bit more stoic).  

I really don't see a reason why the person who gives Digory and Polly rings has to be Digory's uncle instead of his Aunt.  It would feel less sacrilegious to some than the Aslan gender swap and it would give Meryl Streep a more showy role to show off her acting talents.

I also think he theory of her being Mrs. Lefay is interesting, although in that case it would more likely be a cameo than a supporting or main role.