Wizards in the Potterverse can also apparently cast spells all day long; none of them ever complain about getting tired, or running out of magical energy in any way. That makes it a pretty tricky point, for d20-based magic.
There are creation and conjuration effects - Dumbledore has created minor objects several times, mostly chairs. I think Hermione created some rope at one point, too.
Let's see here. What classes do they have at Hogwart's?
Transfiguration would be all Transmutation magic, clearly.
Enchantment seems to be mostly spells based on forces - the summoning charm, levitation... uhhh. That's all I can actually remember from the books about what they've learned in that class. Maybe a protective charm? Seems like mostly Evocation stuff, actually.
Defense Against the Dark Arts is almost all Abjuration. And Knowledge, but that's secondary.
Divination is, well, obvious. With the implication that some people have The Gift, and some don't, and thus can't do divination at all. Sounds like a feat to me. Wonder why they have people study it, then?
Potions seem to vary quite a bit, though they all take some time to make, measured in minutes at least. I'd probably say a minimum of ten minutes to do any potion. It's not clear why you'd use a potion for lots of things, though. Couldn't someone who was good at Transfiguration replicate a Polyjuice potion? Maybe you can't affect yourself with magic, other than potions? I can't think of any instances of someone using magic on themselves that wasn't a potion. You can certainly alter others, but perhaps not yourself? That might explain it.
Arithmancy and Ancient Runes are both unexplored, though we known Hermione takes both of them. Heck, we've never even met the teachers for these subjects. Probably both just specialized Knowledge skills, though.
History of Magic, Care of Magical Creatures, and Herbology are all Knowledge skills, rather than magic.
Personally, I think Potter magic would best be implemented with a skill-based system. Maybe something like the skill tricks that appeared in Complete Scoundrel, with skills based on the different types of magic?