I would love to see a new world, maybe with two different parts...one a livable, spooky town and another part a bustling town square where you can go to little stalls, and stores with people selling their own potions and ingredients, wands, spellbooks etc. Also in the town square there could be a main area where people have duels, maybe play a sport similar to quidditch....... It would be cool if in order to make a potion, you would need to gather your ingredients. Level of difficulty and rarity of the ingredients would be based upon what type of spell you are trying to cast. Also maybe certain spells need to be cast with other witches for more power, or during a certain time of day, or time of year.
It would also be super cool to have dragons. I think that might be unrealistic- but i would love a pet dragon. Also i hope that you can make your pet a familiar if you are a witch or warlock.
In terms of getting to this magical world- a Sabrina The Teenage Witch feel would be cool- where your sim could chose what type of portal they want to have.... closet, basement, etc.
I also really like the feel of Makin Magic... the whole magician thing was really cool so maybe you could perform shows for people and gain fame from how good of a magician you are....
FINALLY.... a little bit of harry potter world could be cool too... and i honestly think they will include some sort of system for children similar to harry potter where they would go to a magical school of some sort- and that would link in really nicely with University, which i think will be the next pack after magic. I wouldn't mind if the town was kinda similar to harry potter- where you have to go to a store and find which wand is right for you, and there could be a store for familiars too.