Unfortunately, the "secrets" system used for necromancers isn't very extendable. The only motive for learning a secret that the game recognizes as valid is immortality, which only applies to historical figures in their extreme old age, and never to goblins or elves. Additionally, historical figures looking to learn a secret only seem to bother searching necromancer towers for potential mentors. So secrets that aren't about necromancy are pretty much doomed to never spread out beyond the first person to discover them.
The new knowledge system changes everything.
Inspired by thriggle's discoveries
here, I created the following secret:
[INTERACTION:SECRET_FIRE]
[I_SOURCE:SECRET]
[IS_NAME:the secrets of fire]
[IS_SPHERE:FIRE]
[IS_SECRET_GOAL:IMMORTALITY]
[IS_SECRET:SUPERNATURAL_LEARNING_POSSIBLE]
[IS_SECRET:MUNDANE_RESEARCH_POSSIBLE]
[IS_SECRET:MUNDANE_TEACHING_POSSIBLE]
[IS_SECRET:MUNDANE_RECORDING_POSSIBLE:objects/text/book_instruction.txt:objects/text/secret_fire.txt]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_REQUIRES:CAN_LEARN]
[IT_REQUIRES:CAN_SPEAK]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[IE_ARENA_NAME:fire mage]
[SYNDROME]
[CE_DISPLAY_NAME:NAME:fire mage:fire mages:fire magic:START:0]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Hurl fireball]
[CDI:INTERACTION:MATERIAL_EMISSION]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:FLOW:FIREBALL]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:30]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Spray jet of fire]
[CDI:INTERACTION:MATERIAL_EMISSION]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:FLOW:FIREJET]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:5]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:30]The result? Over the course of a two-hundred and fifty year worldgen, an elderly dwarf uncovered the secrets of fire, wrote them down, and stored the resulting slab... in an ordinary library in the dwarven fortress of Hammerlearned, open to anyone to come visit. Over the years, numerous scholars have visited the library of Hammerlearned, and some of them have happened to come across the peculiar bronze slab Gravelglow:

In short, if you mod in a secret that does not grant the animate dead ability (and thus send the historical figure off to build a tower) there is a possibility that they will store the slab in their local library. Scholars who visit the library during worldgen may then learn the secret, causing it to spread. Presumably they will eventually write spellbooks of their own and store them at their own local libraries, but 250 years apparently isn't long enough for that to happen.
This opens up a lot of potential for modding in new magic systems.