Vis and ceremonial casting spell mastery.
Another reason I find the original proposition in this thread lacking, is that the gradient of success being described already exists for spellcasting - in the range of possible dice rolls which would provide sufficient penetration against particular amounts of Magic Resistance. You do in fact gradually attain the ability to successfully overcome, say… a Magic Resistance of 23, using a particular spell. And since MR can concievably be any number, there is always going to be the possibilty of a lesser or greater challenge compared to every single spell you can cast.
Now, one might object: wouldn't you just cast a lower level spell where penetration is guaranteed? Or use arcane connections to boost penetration? Yes, you would, because spellcasting is not comparable to ability rolls.
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