To the director:
Please read the book. I think a lot of the fans got the sense Michael Apted was indifferent toward the 'Dawn Treader' book and you can rarely have a good adaptation from a director who doesn't even try to be accurate to it. You need to have your own vision and some liberties need to be taken in film versions, I know. But you should work overtime to make sure your changes don't completely violate the intent of Lewis' novel.

To the production designer/art director:
Make this a world that looks lived in. The first film looked too clean and sterile. 'Prince Caspian' did a better job of conveying a sense of history. Spend the extra effort and make it look convincing. More real set pieces as opposed to green screen would also be best.

To the screenwriters:
Please respect the source material! VDT was a disaster here. Adding little touches to bring it closer to modern movie conventions is one thing, but don't go overboard, especially with the original narrative. And if Jackson and Co. can successfully take word for word passages from Tolkien's novels and incorporate them in to their screenplays, there should be no reason why you can't give Lewis' material the same amount of effort.

The more you change from the books the more likely it is it will fail.

The more you change from the books the more likely it is it will fail.

Not necessarily. Change can be good.

Please find a director that understands and loves the book. Its simple (or at least I believe so), just respect the source material and try to make an adaptation that Lewis himself would love to see.

For the actual filmmakers like cameramen, etc just make sure you shoot a compelling story with the shots, for the director please do a lot of research, this includes the other books besides just the SC, even if you're already a Narnia guru please do it anyways.

I would say.... no magic swords. or green smoke. and keep the characters true. and that I can't wait to see it.

Don't dumb down the script for your audience à la VDT. Treat the story with respect and don't make the film solely for young children because the source material is a children's book.

Keep Eustace and Jill as 11/12 year olds; don't make them 15/16. What was it Polly said in The Last Battle about wanting to race on to the silliest time of one's life as quickly as possible and stop there for as long as possible...

Dear sirs:

Please understand that this audience is going to be well versed in the mythology Silver Chair springs from, even if you aren't. Also, modern film audiences are also READERS unlike 20 years agao - and they always think the book is better.

All that to say, please don't patronize. Make something intelligent. Talk to us like adults.

If you don't understand how intelligent adults can still love a children's story, then go home. Read the book you most loved as a child and ask yourself - even now as your "adult" self - how you would want that book to be made.

Also, get enough sleep and plenty of water and never eat marinara after 8pm.

I'd say first off, have a plan and know what kind of movie you want to make. It's obvious they didn't know what they wanted from VDT. They got the rights to make it and didn't want to lose them.
Just saying to stick with source material and such can only take you so far. You need to have a vision for what you want to create from that story.
It kind of comes down to what story you want to tell. Is the movie and the book telling the same story?...

Get a good script that doesn't make the characters look dumb or immature. Give them good, smart dialogue.
Keep it character driven, not action driven.
I too would like to see more scenery that looks real and not like it's on a green screen. Make Narnia feel like it's a real place, not a dream world.

Very tough questions. I'll try and put down some fairly thoughtful comments, because who knows, the filmmakers may or may not even read this!

For now, here are my two things:

Number one, I would ask that the filmmakers would respect the source material as much as possible. It is true that some things are not as easily conveyed on screen as they are when they are read in a book. I just do not believe that they should justify making deviations simply because they are enjoyable to make.

Number two, try to see the story for its unique qualities. Why is it being made? Is it being made for the potential it has to become 'the next huge Lord of the Rings/Twilight/Harry Potter'? Or is it being made because it is a good story that needs to take place on the screen? Don't try to copy other movies -- it doesn't work. Try to follow in the vein of what Lewis was trying to say in this particular story. Let TSC stand out well on its own. The audience may just come to the conclusion that the movie was memorable enough, unique enough, cool enough, that it deserves several viewings, and even a sequel.

Keep the characters and themes true to the book.

I would most definitely tell them to stick as close to the book as possible. To date, the most successful film was also the one that stuck closest to the book. VDT was changed way too much. The longevity of the books and the popularity they still enjoy its a testament to Lewis' spot-on storytelling, and it would be a shame to see another one botched by over-Hollywood-izing it in an attempt to make it bigger.

I hope they are also able to use the same actors or find ones that are very close. The long gap between these movies makes keeping character continuity quite difficult.

As a very long time fan, I want to see all of them masse (accurately), and I'd love for them to manage it without actor changes.

I once (ages ago) drafted a letter that I was going to send to Walden, but I decided against it. I do think it sums up my response to the OP's question, though:

The Silver Chair has so much potential as a feature film. It has a Sleeping Beauty quest-like story, except that the “beauty” is a man and he is rescued by a girl (one of the things I love about Lewis is his proto-feminism). The characters are colorful and the kids are extremely relatable. With the right pacing, script and visuals, it could be a movie that many movie-goers would enjoy, and come back to see again and again. In fact, I think the story could work a lot better on film than it does on the page. I am not trying to sell a script, I’m only trying to give you some general ideas about the book’s cinematic potential. This could be a truly great movie, but certain pratfalls must be avoided.
First of all, know your audience. The Narnia books are family entertainment. They are magical enough to engage elementary-schoolers, but intelligent enough to engage their parents. Your The Lion, the Witch and the Wardrobe film struck this balance perfectly. Prince Caspian was, in my opinion, a good movie, but it was far too teen-centric to appeal to the elementary crowd, who are a critical element of the Narnia fan base. A Narnia movie must have that magical fairytale feel to it.
How does that apply to Silver Chair? Capitalize on the moments of brightness and magic: Aslan’s Country; the wonder of discovery for Jill (remember she’s as new to Narnia as Lucy was in LWW); the merry court of Cair Paravel; the crisp, invigorating land of the marshwiggles; the enchantment when they first meet the Green Lady (they think she’s benevolent and lovely in that scene); the bustling court of Harfang when they first arrive; the glowing forest of Underland (I think your visual effects people could do an amazing job with that); the bejeweled land of Bism, perhaps; and the snow dance. Also, make sure the moments of darkness have a feel of suspense and adventure, like the dark moments in LWW, rather than the period-piece gloominess of Prince Caspian. The fight with the Green Lady would need to be reworked somehow, so that the movie doesn’t devolve into horror.
As I said before, the Narnia books are both magical and intelligent, thus designed to appeal to all members of a family. Voyage of the Dawn Treader had the fairytale magic down pat, but with over-simplified themes and dumbed-down dialogue, it was more of a “kiddie” film, like the Tinkerbell or Barbie film franchises, rather than a family film. Kiddie films typically go straight to DVD and even those (like Yogi Bear) that make it into theatres never make enough to justify a block-buster budget. There’s a reason for that. Kiddies don’t pay the price of admission, their parents do.
How do you make Silver Chair intelligent enough to satisfy the parents? Remain faithful to the thematic complexity of the book, for one. Of course, you should trim some of the visually duller sequences and embellish some of the more interesting but, unlike Caspian or Dawn Treader, broad changes are neither necessary nor helpful. For another, although some dialogue needs updating for audience comprehension, keep a healthy helping of Lewis’ dialogue in there, especially when it comes to Puddleglum’s lines. He’s hysterical.
So much for the target audience, now on to marketing. The American marketing for VDT made it look like a feeble attempt to milk what was left of LWW’s good will, rather than an interesting movie in its own right. Any future Narnia movie has to be sold on its own merits. If it wasn’t for parents scouring the papers for G/PG films to entertain their little ones over the holidays, VDT would likely have fallen off the map by its second week. In the case of Silver Chair, it would probably help to drop the “Chronicles of Narnia” from the title, since people tend to look askance at sequels that don’t feature any of the original protagonists. . . .
Silver Chair. . . must be marketed as a stand-alone. I’m not saying you should hide the fact that it’s Narnia, but let audiences understand that Silver Chair is its own story, not a follow up to the Pevensie trilogy. Sell the characters. Jill and Eustace are a couple of boarding school misfits who learn to like and trust each other in spite of their differences and initial mistrust. I think it would be best to portray them as middle-schoolers, aged 12 or 13. In marketing Silver Chair, you should emphasize the fairytale adventure too, but the character dynamics are even more important. Those dynamics are what set this story apart from generic mid-grade fantasy.
I think a well-marketed, well-written adaptation of Silver Chair could bring in a lot of casual fans and capture the imaginations of the movie-going public. It might even leave people clamouring for a Silver Chair 2.

That's a good list of points Anhun.

I never thought about Jill being new to Narnia. That could definitely be a fun thing to include in the film. Her reactions could start off more like Susan in LWW, feeling unsafe and distrusting, and gradually become more like Lucy, seeing the wonder of Narnia and trusting.
Same can sort of be said for Eustace, he's never been literally in Narnia, but the world Narnia is in. I hope they mention that in the movie. Eustace most likely has never seen a marshwiggle or a centaur or giants or certain other creatures till then.

The snake/climax scene will definitely need to be reworked. Unfortunately that scene just seems too small of a climax for a blockbuster movie, which they will most likely try to sell it as. Maybe if she turns into a giant snake, like really BIG. lol And instead it being brief like in the book, make it longer, kind of like they did with the dealings with the dragon in Hobbit: Desolation of Smaug. I rather it be close the book, but I can't see them being satisfied with the book's final scene with the LOTGK as is. That will definitely be a difficult scene for them to figure out on screen. :-/