Hagrid as the Hermit, clearly. Walking into the Forbidden Forest with a lantern and Fang.
I see the Dementors in the 9 or 10 of Swords, honestly, not in the Majors. They are too one-sided, not enough room for interpretation in the Majors.
Five and seven of wands as Quidditch matches!
I like Hermione as the High Priestess, much more than Trelawney, who is basically a flake. I can't see the High Priestess symbolized by someone so ungrounded.
Death card--Sirius going through the Veil? After all, we aren't sure that he's dead, now, are we???? But it's definitely a change, a transition, a big one!
I agree with a young Harry as the Fool, his parents as the Lovers, but instead of the Star, I'd like to see Lily as Strength. She exemplifies tremendous courage to me. Not sure about the imagery, though--her defending Harry against Voldemort would be too limiting on the card. Something more serene is called for. Perhaps a young Lily, studying, and petting the monster in the lake?
Dumbledore is definitely the Hierophant! I like Snape as the Chariot. Sybil Trelawney as the Moon? Fudge as the Emperor? I understand what some folks are saying about Voldemort as the Devil, but he fits, he really does. People are forced to make a choice with him--fight on the side of good, and risk death and pain, or give in and go with him, even though he is evil.
I like Firenze as Temperance, mainly because I like his world view. He'd also be a possibility for the Moon. The Wizamgot (sp) as Justice. As others have mentioned, the scene from the end of Chamber would make a great Judgement, and the Weasely twins would be wonderful as the Magician.
It's a little unorthodox, but I like the Whomping Willow as the Tower.
Wheel? World? Star? Sun? Where's Ron? These are things to ponder.