So my friends and I thought about ways to make magic more interesting, especially learning spells, especially the ones that every spell caster gets by default, such as invisibility, fireball, summon spells, or whatever. We thought making it so you had to collect certain spells from magical creatures such as dragons, lamasus, demons, angels, etc. could be interesting.
So I'd like your help to present some of those too common/powerful spells and/or creatures that could teach you. And maybe ways to locate said creatures or what they might want in return.
Example:
Lammasu; teaches greater invisibility and dimension door; they want wisdom and learning; they will only teach those who prove themselves good-hearted and just;
Legion Archon; teaches greater teleport; they want to know how you have spread peace and mercy in the world; they want help defeating some demons that are reeking havoc nearby;
Imp; teaches invisibility and commune; they want you to pull a vile prank; they want your soul;
Green Dragon; teaches dominate person; they want gold and treasure;
The problem I see is that this makes learning spells from new books virtually impossible.
Well you can still learn most spells on your own, like cantrips and most lower level spells, and could even potentially research any spells as if creating your own spell. In the section under creating new spells it talks about benchmark spells that stand for the most powerful of that level. It's these kinds of spells that would have to be learned specially to reduce their availability and encourage the use of other spells.
There are many methods of learning new spells. Learning them from magical creatures, that could work too.