This article states that Liam Neeson, the voice of Aslan in the recent Chronicles of Narnia movies, claims that Aslan could be any religious leader.
Simply put, that’s not the intention of the author.
Perhaps with some creative cutting you can twist Aslan to be a generic religious hero/ leader in a few of the books (for instance, in Dawn Treader it might be possible). But in the most famous of the Narnia books, The Lion, The Witch, and the Wardrobe, Aslan dies as an innocent sacrifice to take away the guilt from a human malefactor.
Aslan is a figure of Christ, a picture to help us grasp what Jesus did for us. That’s what C. S. Lewis intended, and it’s impossible to twist that. What other religious figure died to free others from their guilt? Certainly not Muhammad. Not Buddha. No, only Christ takes our guilt away. Every other “religious leader” tells us to take care of our guilt ourselves. And that’s Lewis’s intention: he wants to show us Christ. Like Brandon’s post mentions, the point was to glorify Jesus.
Not “religious leaders.” Not “spiritual pioneers.”
To say that Aslan could stand for any “spiritual leader” misses the point of Lewis’s books entirely. Maybe the actor playing Aslan should go read the source material a few more times.