On Pottermore we learn that Hogwarts may offer classes such as Alchemy if there is enough demand for it, so I thought we could do same in the RPG. I think it would be interesting and a lot fun if each year we could have one or two additional and different classes added to the students curriculum. We wouldn't need any additional professors for this, though.
For example, Alchemy could be offered and two professors (Potions and Transfiguration) could come together and teach at least one class on the subject.
Other examples are Conjuration (I know it's a branch of Transfiguration but it's more specific), Healing (Healer could teach this), Wandless Magic, Legilimency and Occlumency,
Dark Arts*, Elemental (you know, how Dumblz and McGonz could summon fire and control it like a boss -- minus the conjuration part), Magical Beings (Vampires, Werewolves, Veelas, Goblins, Hags, etc.), Parseltongue? (or any other animal tongues?), and something like object making, such as
horcruxes* wands, rememberalls, portkeys, couldrons, etc.
I think this would be a lot of fun for students, and hopefully for Professors, too, but it would require a lot of imagination and creativity.
* = 