I will suggest (just in case you hadn't considered them yet):
- The harder the quest (the thing that holds the good witch from fixing the problem/defeating the bad witch) is, the more interesting it'll be for the reader.
- The bad witch needs to appear undefeatable.
- You have to setup the good witch to be likable or the reader won't care about them or care if they succeed
- The bad witch needs to have a logical reason for doing what they're doing (they can't just be bad because they're bad--or you'll have a 2 dimensional character that will seem unbelievable)
Good to see you're doing your "writing" before you start actually writing! Sounds like you're having fun!