Something I missed in Player’s Option: Spells & Magic is where the costs for optional abilities (p. 24) can be reduced by delaying when they take effect (i.e. what level, between 3^rd and 11^th). Do this with abilities and limitations and you could design a spell caster where magic changes in nature for the character over time. Talk about a new tool for the toolbox, this could be huge if used right for your settings. Here are a few examples:

One ability is to take spheres as new schools (and the reverse in the priest section), if a wizard can do that at 11^th level, then the sphere only costs twice as much as a school in character points, a bargain if the sphere is one of the large ones, such as Plant, Time or Thought. Thought can bring so many possibilities to an arcane caster. But to get that ability to manipulate minds (one’s own and others) the character has to pay for it with a limitation. My favorite is supernatural constraint; something the wizard can not do because the universe will not allow it. In this case, with the ability to manipulate minds of intelligent beings, the wizard loses all capacity to understand creatures with an Intelligence less than 3. Horses are difficult to ride because the character can not pick up on any clues that the beast is giving, former pets are no longer of interest. Speak with (Plants/Animals/Stonetell) is useless to such a character.

A magic-user gains access to the Sphere of Water at 9^th level, but loses the ability to memorize and cast fire spells when on or under water. Any attempts to do so results in a fizzled and wasted spell.

When a 5^th level wizard wants to extend the duration of their spells, they can do so by manipulating their own time stream. In effect they become Slowed for as long as the spell is active.

Another wizard, who is 7^th level, has found a way to extend their spells, be she can only do so when the star Algol is in the sky. The spell is extended by the alien godling living on a planet in that system.

Pious wizards can gain access to one major sphere (except the one that holds all the elements) if they are in the presence of an idol of their god. No idol, no ability to memorize the spells from the sphere.

Rods of Power are devices created by the magic-users that wield them. The casters have to be 3^rd level to do so. When the caster is holding their rod, they gain additional hit points as if they had a 1d6 hit die (1d6 at the first level they create the rod and additional 1d6s at subsequent levels). These points vanish when the rod is no longer held. And there is another downside- the elves were the first ones to develop the rods and one of the orcish gods cursed the process. Now if the caster sees orcish writing, they forget all of their memorized spells whether or no they are holding their rod.

One crazy 9^th level wizard found a way to blast distant foes by channeling magic through their body. He has a 50% range increase for all of his spells, but using the extended range inflicts one point of damage per spell level (a save versus breath weapon negates this damage).

Magic-users that join one particular army learn how to cast spells in armor. They can wear padded at 3^rd level, other non-metallic at 5^th level and any metal at 7^th. The downside is they can’t cast spells when they are not in their armor. Even the loss of their shield or a gauntlet renders their powers null and void.

Those wizards who want to gain personal powers can do so in a few different ways. Those who obtain a spell such as Fly, Iron Mind or Wizard Sight (i.e. 3^rd level) at 7^th level have to give up the use of all magic items except artifacts and relics. The power within themselves interferes with the magic within the items.

Another wizard gained the use of Polymorph Self at will at 9^th level, but can not longer use any non-natural weapons (doing so costs them their spells for a month), visibly warps when casting any spell (this does no harm, just makes them really obvious as a magic wielder to anyone watching them) and their spells can not harm any creature that the wizard mimicked. So if they turned into a minotaur one time, their spells can not affect any minotaurs thereafter. Such is the price of power.

What makes these and anything similar evolving is that the player has to select what their characters get and when during character creation. They have to look forward to the higher levels of the character and decide what fits their envisioned development for their spell caster. Now I wish I had picked up on that paragraph years ago.