Would Saruman Be a Great Narnian Villain?
No. Don’t even go there. If you want to write Narnia fanfiction, write stories which fill in gaps or which explain inconsistencies. Maybe you could have an old wizardly guy coming to Narnia or Calormen or Terebinthia or Archenland or Telmar some years after the 4 leave their Golden Age and stuffing people around. Maybe a star who comes to Earth for a sojourn but gets too involved. He could have a similar character but please don’t have him called Saruman or have him come through a portal from Middle-earth. And if he is into the domination of a specific place, you would need a clear motive and reason for him to be bothered about that specific place. Not even Jadis had that nor the Green Kirtle woman… not when there was a wider world to dominate.
He Could Make a Good Villain Anywhere
He has all the arrogance, selfishness, and greed needed to make a good villain. However, personally, no he wouldn’t. Assuming that somehow he could reach that wood with its pools and went through the correct one to reach Narnia, he would find that Aslan is quite prepared to directly intervene if something is being truly wicked there.
The nearest individual to Saruman in The Chronicles of Narnia is Jadis, the White Witch. Even her killing Aslan did not stop him defeating her the next day in open battle. Saruman is indeed truly powerful and at the level of Jadis, I agree, but Eru lets the song work out and works behind the scenes if he can. Numenor was an exception. Aslan is Aslan, and He is not a tame Lion friendly most of the time but not a housecat. So somehow Saruman would be put back into Middle-earth, and the Valar getting a polite note to keep an eye on their staff.
The Complexity of Saruman
Could Saruman's character cross over to The Chronicles of Narnia? I don’t think so. Personally, I think that Saruman and the evil he falls into is too mature for the Narnia audiences. Narnia is a story for children. Its characters are far more simple than those of The Lord of the Rings. I feel that C.S. Lewis wrote in a more black/white way, not meaning race here, where Tolkien wrote with many different shades.
Taking Saruman’s character, he was an ancient spirit being who was at the beginning of creation. He takes on an actual human form and comes to ‘help’. However, his motives and actions are deeply complex and fraught with moral shadows. This would make him a haunting presence in Narnia, one that could not be easily explained or understood by the children who are the main audience.
If you decide to explore this idea in fanfiction, remember to ground the story in the values and themes of The Chronicles of Narnia. Aslan is a figure of immense power and wisdom, and he would likely handle any intrusion from an external villain with ease. Any attempt to bring Saruman into Narnia would need to be carefully crafted to not overshadow the heart of the story.