Feature suggestion: A configurable option that you can't pick an enchantment until you have "learned" it randomly. So for example, you can't pick Silk Touch for a pick until you've gotten it at least once as a random enchant. this would give an element of "learning" the enchantments that is missing from both the vanilla system and the current mod.
Also, even without knowing a rare enchant, you could increase your chances of getting it by picking a common enchant at level 1 even when you have a lot of spell levels available. Vanilla would probably give you efficiency III or IV if you have a lot of spell levels, but if you force Efficiency I, you would increase the # of levels available for other enchants.