[PTTG]

There's two kinds of prerequisites: spells and conditions.

Spell prereqs tend to be "if you can do this, then you can also do that, so pay the points for both. If you can regrow a limb, then it's reasonable that you can restore a few hit points.

Conditions are more distinctly limiters built into the way you cast spells. Minimum IQ 12 to cast a spell implies it's too complex for most people. Minimum magery levels mean that the spell simply draws so much power (or requires such a direct magical connection,) that only great mages can use it.

On that note, I wanted to put down some broad advice. If you want to have stringent restrictions on a class of magic, don't make the limitation be "you have to do this involved adventuring task alone in order to unlock this power," unless that option or similar ones are available to everyone.

If the party's wizard (and only the wizard) can monopolize hours of game time and thereby get more power than everyone else, and use that to monopolize more game time, you're going to make your warriors and such feel left out.

Contrariwise, having single-character spotlights of some kind open to each PC is a great idea.