I wonder if you can't combine the idea of a mana pool-like system with a memorization system. For example think of the idea of the more potent spells as those that require memorization, that you only get a very limited number of uses from, however, combine that with a second class of spells that use a mana pool-like system, that are much, much weaker, but can be used more consistently,
You could definitely make use of a specialization oriented system that suggested you needed to either branch out, or specialize, depending on your play style and goals for your character. However you'd specialize your weaker mana pool-like spells separately from your memorized spells. They might share a school system, but they'd cover different realms of that system. The Memorized spells would be your bigger, more complex spells. The mana pool-like spells would be simple spells that at their best might compare to cantrips, while still putting a limiter on them like a slow regenerating mana pool-like system. Though I'd prefer a small mana pool to the large ones.
You could take the second tier of your poll, and, again, use both. Have the very specific styled spells be your memorized spells, specific named spells that you can only memorize so many of at a time. In contrast your mana pool-like spells would use the upgrade of older spells. In my mind this means specialization would be even more limiting for the mana poo-style spells in this context where you have both. Because the memorized spells would be your variety, even if you've specialized in an entire school, but your mana pool-like using spells would be limited to one or two spells that you could highly customize and upgrade to your needs. Think of these cantrip level mana pool-like spells as being like your character in that you'd need to level them up toward different functions.
Perhaps your cantrip-like spell is a little ball of electricity. At a base you launch it toward an enemy and it does very little damage. Now, you could upgrade its damage to an extent, but your memorized spells would always do far, far more damage (or be better at any given/equivalent) effect we're talking about, we're just using a damage spell in the example). However you could also customize the ball of electricity to split itself when you launch it or to become a swarm of projectiles or to track enemies or explode or pierce or what have you. You'd only be able to upgrade it so much as you leveled, but you'd be able to customize it and combine upgrades to an extent to your character's needs.
Regardless they'd always be weaker than memorized spell equivalents, and used not as a main means of attack (or whatever the function of the spell) but as a sort of natural wand. In fact I'm not entirely sure the system should be, 'those one or two wands (or a staff, or whatever you're thinking) you're taught to craft that have a slowly regenerating power supply in them' that you can upgrade and augment as you gain levels and different forms of upgrade are made available to you. You could treat them as weapon sets, and prevent you from equipping new equipment in battle, but allow a character to switch between those two weapon sets letting you change between two of these effects. Obviously much more limited than Memorized casting, but pointedly so.
In contrast your memorized spell of 'the big squishy energy fist of doom' would be a named, already customized - by another mage long ago - spell that you have to memorize. You'd memorize charged of it much like in DnD. The spell's function would always be the same. However it would also be much more potent, and always be so.
I'm not entirely sure you have to go with an Arcane/Divine model at all . . . what if you made more base distinctions than that? I always felt, for example elemental (obvious) from arcane (the more wierd non-elemental effects and spells) and divine from nature. Regardless I'd want there to be much less cross over than you saw in DnD. I'd definitely want spells in one area to be wholly unique from spells in another, especially where memroized spells are concerned.
Edited by Umberlin, 19 September 2012 - 12:53 PM.