Interesting idea. I'm still a little wary of going off-book with these new adaptations, but it's a fun thing to think about.

I could see there being a story around how the evil creatures (e.g. minotaurs, hags, efreets) came to live in Narnia in the first place since they weren't present during the creation of Narnia in MN. It makes sense that they weren't present because it would imply that Aslan created something evil. That would probably also rule out the Tolkien idea of having them be corruptions of good creatures (like the orcs are corruptions of elves).

The most obvious solution to me is that the White Witch summoned them from another world. What kind of world might they have come from? Maybe a world that has become even more corrupt than our own.

As for Aslan's involvement in the resistance to the White Witch's takeover... I think it might be best for him to be offscreen, absent like in PC. Otherwise, you would have to contrive some way for him to be defeated by the White Witch and I don't know if you could do that and remain faithful to the spirit of the books.

@reepicheep775

This is very true. I can't think of a practical way for Aslan himself to be defeated by Jadis. However, one thing Nikabrik says in the Prince Caspian film (as well as the novel, I'm sure) is that Jadis' power was "one that kept even Aslan at bay for near a hundred years", which implies she had some form of control over Aslan at one point. But then again, this could just be a perversion of the truth shared by the black dwarves and other Jadis-sympathisers over 1,300 years in order to further their own agenda; perhaps it was actually the will of Aslan to wait a century to come back to liberate Narnia from the Long Winter, maybe he waited until the Pevensies came for the sole purpose of teaching them valuable lessons?

Interesting idea!

I think that Nikabrik's point about Jadis' power "keeping Aslan at bay" was definitely revisionist history on his part...most likely because it was the best fit for his embittered and jaded outlook on the world. 

For Aslan's timing, I think that teaching the Pevensies lessons was part of his reasoning for finally liberating Narnia when he did; I think he also wanted to teach the Narnians themselves through the experience as well.  Not only the liberation, but also through the Golden Age, and all that would happen over the centuries.  While some would retain Nikabrik's outlook, others such as Trufflehunter remained faithful to Aslan, and remembered his actions + the Pevensies' legacy.  

It would be interesting to see some of those gaps filled in through original material.  A little risky, though!  I personally have my reservations about it, but would be intrigued to see what people could come up with!

It's an interesting idea, but my biggest concern about it is one that someone else raised recently in another discussion (I can't remember where)... just the thought of someone other than C.S. Lewis writing Aslan. I mean, of all the characters that it's hard to "get right" when writing a continuation of an original author's work, the character who basically is God / Christ has got to be the hardest, and the most sensitive and the most controversial to try to do. To not include him at all would, I think, make it somehow not a "real" Narnia story — Aslan is the one character who features in all seven of the books — and yet if they did include him, no matter how they chose to portray him, I can see it being a huge source of contention between fans. (Is this really how Aslan would speak and act? What theological message are they trying to imply here, which maybe not all viewers will agree with? Would Lewis himself ever have approved of this?... and so on.) And then, yes, there's the difficulty of having Aslan either being defeated or choosing to retreat and leave the Narnians in the grip of the Long Winter for 100 years. Which gets us into even more tangles, not only plot-wise, but theology-wise. I just can't see it working, at least not in a way that would satisfy anything like the majority of Narnia fans. But it's intriguing to think about!

EDIT: I've just found the earlier discussion where this issue came up (the problem of someone other than Lewis "speaking" for Aslan) — here's the link.