I've been working on a pair of tables to be used by magic-users and clerics, respectively, when their spell slots run out. Lately, I've been thinking that the standard spell progression stuff covers what a magic-user or cleric can safely cast. Once their spell slots are expended, they can still attempt to tap into arcane forces or call on their god for aid, but what happens as a result is all up to fate.
I'm trying to make each table distinctive, representative of my take on each of these classes. For the magic-user, harnessing magical forces beyond their control is pretty much an all-or-nothing scenario. It either helps them or harms them; no matter how minor or major that effect is, it can never be neutral. On the other hand, my cleric table has a wide 8-12 space consisting of nothing but 'your god does not hear your prayers.' It's much more of a bell curve, with your god striking down great vengeance at the top and smiting your needy ass on the bottom.
Resulting product will be forthcoming for analysis and criticism as soon as I finish with 'em.