The first step is probably to figure out which part of natural spell is the "broken" part. Is it the ability to buff your animal form? (If so, perhaps it makes more sense to depower druid buffs, and natural spell will be weakened as a side effect.) Is it the ability to use animal form for defense while casting? (If so, perhaps you want to make it harder to combine animal form with normal defenses.) Is it the ability to switch between physical and magical attacks without needing a standard action, and switching unlimited times a day? (I doubt it, as you're probably not going to want to switch that much so often anyway.) Is it the ability to attack with natural weapons in the same round you cast a (nonquickened) spell? (If so, I want to know how you pull that off.) Is it something else? (If so, what?)