I think DADA is a popular subject among Harry Potter fans and in terms of characters it's something for everyone really. The death eaters like learning about the dark side within the subject then the good guys like learning how to defend and fight the dark, plus it leads to a career in becoming an auror.
HERBOLOGY PLEASE! No seriously you don't understand how much I would love to role-play in that lesson! (I will make a professor character to teach herbology if needs be!
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Divinations would be a fun class to role-play in and I think, as Jack said, a teacher who can actually predict would make for interesting plots and that could, in turn, get the teachers more involved with plots!
I think only have one of charms or transfiguration? I know they're different but in a way they are too similar just learning spells. Probably charms because we actually know more charms from the books and films than we know transfiguration from the books and films (conjure, vanish, switching spells and then a lesson on animagi? Other than that we don't know much about transfiguration).
Also with charms, JK always said that was a good class to talk in because there was so much going on so Harry Ron and Hermione used to talk about stuff. Maybe if students did stuff like this between them to, referring back to their own plots and recent threads in a lesson like charms it'd make the whole lesson experience part of their roleplay as opposed to just a lets-get-this-over-with kind of thing.
Potions is a good lesson I think because it had the practical essence so I reckon we should keep that, and I guess it ties well with Herbology and the plants that they learn about. In addition it's always fun.
COMC. I feel like the only reason to keep this would be because there are a number of characters who like the subject and on PA the department of magical creatures is one of the main departments so it leads on well. Although, I don't know. It can be a bit boring depending on the creature and the activity.
Astronomy would be good to do like the odd lesson or so as a change, and we already have the astronomy tower but I don't know if people would actually attend that class.
Then yes, do an odd apparation class and flying class
OH! It'd be cool if the teachers came in and out of our normal role plays. I know most people like private topics but it would be good to have more open topics where anyone can go into and you don't know who will appear in what topics, much like you don't know who will walk round the corner in the actual Hogwarts castle?
Back onto lessons. Like you said the lessons need to be fun, even if the teacher just sets a task if the students put zest in or something big happens like in potions Henri caused chaos and made the thread more fun. It doesn't always have to be chaos but in DADA we could pair up or give each character the chance to practice with a boggart to get the RPer thinking more about the characters fears and incorporating that into the roleplay.
I used to teach herbology on another site and it would rather fun, or at least I thought so.
Muggle studies would be boring. Just saying. Don't go there.
Umm. I think that's all my views haha.
DADA, Herb, Div, Pot, Charms