A refined version of the magic as you rise in level wish.
#xpmagic <mask> <percent>
So a theurg acolyte could have a 30% chance of gaining AWS every time he goes up in level.
A theurg could have a 20% chance of gaining AWSF.
And a an archtheurg could have a 10% chance of gainin AWSF.
It would be theoretically possible for lower level units to "grow" into the higher level spellcasters, but rather unlikely (still, out of fifty theurgs you recruited, one would probably get the three bonus picks and become an archtheurg equivalent.)
And most of the high level casters have a 10% random anyway, so you'd just get another shot at that random each time they went up in level.
Edit: I'm not suggesting that this feature should even exist in the basic game, just that it should be available to modders who wished to try it.