Magic Is Magic

I've been thinking that magic ought to be magic. No, I'm not talking about making magic more magical, a topic that shows up from time to time on blogs and forums.  I'm talking about the idea that Cleric spells and Magic-User spells can be treated the same, and that theoretically a Magic-User can learn a Cleric spell, or a Cleric can learn a Magic-User spell. It's just their methods of preparing and casting the spell that are different.

This doesn't throw the spell lists wide open. Clerics by default know all the spells on the Cleric spell list. They don't know those on the Magic-User spell list, and must research how to cast them in the clerical way. Any Magic-User spell is treated as a reversed Cleric spell; Lawful Clerics can only cast these spells in dire circumstances or they risk falling from grace.

Magic-Users, on the other hand, must worry about alignment when casting Cleric spells. Magic-Users who turn Chaotic can no longer cast healing spells, although they can prepare healing scrolls for other people to cast. Neutral Magic-Users can't cast 5th level Cleric spells at all, nor can they even prepare them as scrolls.

Other spell-casting classes normally have a spell theme (nature spells for Druids, mind/illusion spells for Illusionists) and can't learn spells outside that theme, although they might be able to create themed equivalents of those spells (Raise Dead as Tree? Illusory Cure of Wounds?)