I'd like to introduce a new concept for my spellcasters, and I wanted to get some opinions on it.
I want specialist, and those who have the Spell Focus (school) feat, to have the ability to select any spell from their chosen school, regardless of class.
So, a conjurer could learn any conjuration spell, even those unavailable to wizards. This would also mean a cleric with Spell Focus (necromancy) could cast necromancy spells unavailable to his class.
I wouldn't though a feat allowing the learning of spells outside of their class might be fine, the pathfinder savant has a similar ability to pick a single spell every level in the PRC after 1st, that spell is added to the class spell list at 1 level higher.
If it is of the same type of spell list (arcane/divine) apparently it does not increase, though I find that rule shady and possibly abusive with new classes coming out with new spells, the 1 level higher is probably fine.
I think an archetype might be very suitable for this purpose, a religious wizard might get some divine spells as arcane spells from a single domain, a cleric of a deity of magic might sacrifice their domains to get access to some arcane spells as divine spells, or it might function similary to the pathfinder savant prc.
How about a feat that allows a specialist with Spell Focus to be able to learn a handful of spells of that school from another list, and then, this feat becomes a prerequisite for a prestige class through which the character eventually learns more and more such spells (starting at low levels but going up quickly) until, at 10th level, the character has access to all such spells?
Or, something like that.
Well I am not really fond of it, making it too easy cheapens the different classes, you noticed that most classes are spellcasters of some kind.
Spell lists are a big part of what makes a witch unique for example, also you notice that some spells that are on the wizards list are actually 1 or 2 levels lower or higher than the cleric list, it does not stroke well with allowing a formerly blocked spell entry at a lower level.
I'd take a look at the PFSRD pathfinder savant on how to create such a PRC or archetype and make it more focused towards a single school of magic, I would suggest your other schools would become particulary restrictive in exchange for further specialization that crosses over into the realm of the divine.