Okay, here's something I'm considering for a campaign...
Removing the split between Arcane Magic and Divine Magic. Or, as I like to call it, the "Shadowrun Approach". Basically, Magic is Magic, all of it stemming from the same source of otherworldly energies, that can be shaped by the will of casters to create different effects.
The only real difference between a Wizard and a Cleric are the mental approach of the different traditions: Wizards believe that magic is shaped through complex formulae. Clerics believe that magic is a gift of the gods. The Former believe the latter are just shaping the same energies through rote formulas that they attribute to "higher powers", while the latter believe the former are misusing divine gifts from patrons they ignore.
(Incidentally, I'll admit this concept encourages a more "agnostic" approach to divine spellcasters, withwhether or not they're actually empowered by gods to be left unanswered. But hey, it worked for Eberron.)
The mechanical changes this concept would leads to include allowing arcane casters to use healing magic such as Cure spells, although I would balance it out by raising the Level requisites for them. So a Cure Light Wounds would be a level 1 spell for a Cleric, but a level 2 spell for a Wizard. And any caster class could use a Wand of Cure Light Wounds or a Wand of Magic Missile.
Now, I'm well aware Purists would call Heresy at the notion of allowing Wizards to step on the Cleric's toes, but I've never been a fan of Niche Protection. Besides, even with Wizards having access to healing spells, Clerics still have spontaneous Casting of healing spells, and Positive Energy Channeling, making them the BEST healers, not just the only ones. And that's without counting their ability to handle melee better then Wizards.
Why not just dump wizards, clerics, and sorcerers and use Monte Cook's Magister. Do a complete sweep instead of a half measure.
Because I still like the different flavors of those classes. I don't see Sorcerors being the same as Wizards or Clerics or Druids, and vice versa. They may all harness the same energy (Magic), but they do so all in different ways.
You've pretty much thrown away the differences between Wizards and Clerics, why stop there?
Right, because "Wizards can cast Cure spells, just later then Clerics do" automatically means Clerics lose all the other elements I mentionned: "Clerics still have spontaneous Casting of healing spells, and Positive Energy Channeling, making them the BEST healers, not just the only ones. And that's without counting their ability to handle melee better then Wizards."
Because, as we all know, the Cure Spells are the ONLY thing Clerics do.
Personally I do not like wizards and clerics already since they are too generalized, especially wizards are already too much capable of doing anything, adding cure spells cheapens magic overall I think.
I would reinforce class restrictions, and simply remove clerics and wizards. A witch could easily serve as either a priest or a magician for example and is much more thematic you could argue that witches are very similar to druids already.
You might want to remove special rules for armor casting as well, having existing divine casters cast in lighter armor without fail but using feats to increase that ability perhaps.
Armor proficiency light, medium, heavy
Arcane armor proficiency light, medium(light shield), heavy(heavy shield)
An oracle should probably be proficient with light armor and be able to cast in it, possibly gaining proficiency to cast and wear in heavier armor later, much like magus does.