I was thinking, what if there wasn't a magic tree for witches? Instead, they had a bunch of skills based on fields of magic, and through study and practice, leveled up and unlocked new spells? There would be Dark and Light Magic Skills, meaning you can create an evil or good witch by virtue of what you choose to study.
Of course, I'd still like a skill tree to build on all of that. It could unlock some spells, give you more energy to cast spells, improve potion/spell effectiveness, or maybe even unlock entirely new magical skill groups entirely.
