If she was young I would've said Susan.  I've thought about this for a while and think she could portray Helen or Jadis. She''s got a regal and sophisticated persona every time I see her in interviews so I have no doubt she wouldn't be able to take up a role of royalty. Another role that's been on my mind even though it's a male role but there's been gender swaps and BBC Narnia had done it before with Trufflehunter, but I was thinking if it were made possible it would be Strawberry/Fledge. But I know some or most will probably disagree with me on this and believe me I can take it.  

I had a great thought. If Greta Gerwig does Prince Caspian, Ronan could play the math teacher from Chapter 14. (Not Miss Prizzle. She was a history teacher, remember?) 

@col-klink 

I haven't thought of that one, though I think the Focus on the Family Radio Theatre adaption did a composite of Miss Prizzle and the other teacher. So I wonder if that would be possible if Greta Gerwig does Prince Caspian.

I just saw a video of Saoirse Ronan and her Scottish actor husband Jack Lowden on Twitter and the thought of them playing King Frank and Queen Helen popped into my head! It's the sort of thing I could imagine Gerwig doing and I think Lowden has a face that would work well for both a cabbie and a king — and the same goes for Saoirse as a cabdriver's wife who becomes a queen.

@rose I have thought this before as well, they could even keep their own Scottish and Irish accents since it is clear in the book that Frank and Helen were not Londoners. Jack Lowden worked with horses in War & Peace too so that, in my opinion, is another point in his favor to be considered as Frank in a potential adaptation of The Magician's Nephew.

@narnian-in-the-north 

Not being Londoners would mean they came from a nearby county, not that they were all the way from Ireland. Sorry but this doesn't create a valid reason to cast someone with an Irish accent. 

Posted by: @coracle

@narnian-in-the-north 

Not being Londoners would mean they came from a nearby county, not that they were all the way from Ireland. Sorry but this doesn't create a valid reason to cast someone with an Irish accent. 

There were huge numbers of Irish immigrants in London during the late 19th Century and early 20th Century (largely as a result of the Famine in Ireland in the mid-1800s) with immigration particularly dense in areas of North London where my family is from.

Therefore I wouldn't think it would be unrealistic at all, particularly for labouring classes such as Frank and Helen if they were Irish immigrants.

That's interesting about Lowden already having experience working with horses, @narnian-in-the-north! I never imagined Frank and Nellie as sounding like they were from anywhere but the English countryside, but I think Saoirse keeping her Irish accent would actually be a lovely nod to Lewis's own heritage. Seems fitting that the first rulers of Narnia would be Irish and English, since Lewis was Irish and spent most of his adult life in England!

Ooh, another one to consider is Digory's mother! Very much a cameo, but hey, she would do a lovely job.

Sticking to MN and LWW, (presumably the two Gretta is likely to be making) the roles she could play are:

Various servant roles

Digory's mom

Polly's mom

Aunt Letty (too young)

Helen

Jadis

Jadis' sister

Naiad or Dryad

Mrs. McCready

Mrs Beaver

Mrs. Pevinsie 

Older Susan

Older Lucy

@icarus I'm doubtful that Frank would have been able to take his horse to London from Ireland. If he was escaping from famine, he'd have very little left before he journeyed to England, and how could he have paid to get his horse across the Irish Sea? It's a few hours by ferry nowadays. 

I stick to his having a West Country accent or being from Kent or Sussex. His wife would be the same.