Lets start with a fair warning, there's quite some potential for this thread to turn into yet another martial-caster disparity debate. This isn't my intent but due to the nature of the topic I'm practically obliged to refer to it for a variety of reasons. I don't want that to overshadow the thread's true purpose. When it does I'm out because those debates are tiresome. The debate does have merit so feel free to continue when that happens. I just don't want to get into it myself.
So! On to the actual purpose of this thread! The idea is to give spells prerequisites, like feats. This can take many forms. Here's the possibilities I thought of so far.
First off, the number of spells we give prerequisites to should be defined. All spells except cantrips, all spells except 1st level spells etc. Another is to give a select few spells prerequisites.
Personally, I like the idea to give all spells above 1st level prerequisites.
Second, what are these prerequisites going to be from the generic to the specific. For instance, spells could simply require a certain amount of lower level spells from a specific school based on the spell's level in order to be learned. This is quick and easy and wouldn't take up much space if it were ever to be printed in a book. In the same vein, we could use descriptors so that Lightning Bolt needs a particular amount of Electricity spells. This would require more space, though not that much if it would be a general rule. The most space-intensive method is to give spells individual prerequisites. This method has the greatest range of options available from the same generic prerequisites to feats, skills, abilities, BAB, Base Saves,Class features, races, you name it.
Personally, I'd love to give each spell individual prerequisites and use all the options available. I realize this is messy and unfriendly towards new players trying to grok it all, which is sad and I'd really rather not but I cannot have my cake and eat it too.
So why do this? There's quite a few reasons, really. First off, and here it comes, it can help limit the ease by which casters step on other classes' toes. For example, if Invisibility would require a certain amount of ranks in Stealth it makes it that much harder but not impossible. In some cases you could even use class features that these classes do not inherently possess forcing them to diversify and multiclass in order to access certain spells.
Next, in the same vein, because many resources, like skill ranks, are limited, it makes it harder for casters to 'do everything'. It forces them to make meaningful choices. I don't think it will, or should, make 'God-Wizards' a thing of the past, but it will diminish their ubiquity. A nice bonus, in my mind, is that it gives many casters more of a theme or flavor.
Third, an oddity of such restrictions is that it actually creates more design space. Currently spells should conform to a powerlevel according to their spell-level, but now you can create spells with an eye towards the difficulty of attaining their prerequisites. As an alternative, or maybe in addition to, having more room for spelldesign it can give room to design feats, classes, features, archetypes or races with these requirements in mind. Take gnomes for instance with their penchant for illusion magic. What if they could ignore (some) requirements for spells in that school?
I made this thread with wizards in mind. I think it shows. It makes a lot of sense, to me anyway, to require them to fulfill certain conditions representing them unlocking some arcane secret through, well, whatever it is the prerequisite requires, be it knowledge, devotion, experience or whatever. Its harder to justify spellrequirements for many other classes, especially those who are already limited in one way or another. I would suggest giving most classes some way to circumvent certain requirements, via their bloodlines, deities, patrons etc.
So, there you have it. So far anyway, thoughts are still cooking. What do you think?