Currently, magic is very heavily tied to necromancer towers - it seems nearly impossible to have a secret that is not a necromancy variant to be spread around in worldgen. While this will no doubt pick up during the Myth and Magic arc, it would be nice to have simpler, placeholder methods that could work in manners similar to necromancy or the hard-coded mundane knowledge, but without the requirement for a tower, for modders interested in making a "high fantasy" world, or at least worlds in which there are wizards who don't make zombie armies.
Toady does like slipping extra bits here and there, but given that Myth and Magic is apparently next up after the steam release, I expect he won't be looking to add a placeholder but rather design an expandable and generalized system. A lot of the existing but not usable as tags goals actually require a lot of additional world gen work to be made to work as secret goals in a fully developed way...
Still, the current goal framework could be a good starting point, even if it is a bit limited. I think every variety of secret could have compelling goals. An additional token IS_SECRET_GOAL_SECONDARY or a an additional numerical argument to IS_SECRET_GOAL indicating the priority/alignment of the secret with the goal. Additionally, goals should have interrelations to other goals (similar to how sphere have related, opposing, and parent/child spheres). Each goal should also have things in world gen that can drive historical figures to shift to that goal or to pursue that goal extra hard in order to seek out a secret. Finally, goals should tie into mundane actions as well.
PROSELYTIZE if the secret is good for building faith or spreading the word of their god or out-competing rival religions. Related goals will vary by the domain of the god. This goal could also drive mundane actions that got added to religious leaders in the villain arc.
HEAL_THE_SICK for medical secrets. In world gen, plagues, magical syndromes, and seeing wounded die of infections should make this a driving goal. This goal should drive mundane medical knowledge acquisition and use. This goal can be related to BRING_PEACE_TO_THE_WORLD and PROSELYTIZE as it can serve to advance either goal. If a plague is threatening the historical figure's civilization it can be related to MAINTAIN_ENTITY_STATUS. World gen will need proper plagues and disease spreading to make this goal make sense.
EXTEND_LIFE as a more moderate version of IMMORTALITY. It should increase age but not grant full immortality. IMMORTALITY would be a related goal so that a would be necromancer might temporarily settle for a longer life, or someone just looking for a little bit longer might go full immortality.
SEE_THE_GREAT_NATURAL_SITES tied to spells that allow for faster travel, long distance awareness/scrying/clairvoyance, and extra planar travel. EXTEND_LIFE would be a related secondary goal because a longer life allows for more time to see things. Worldgen will need to respect faster travel methods and account for figures that use these abilities.
BECOME_A_LEGENDARY_WARRIOR any spell with good combat applications should at least have this as a secondary goal. Perhaps historical figures that suffer defeats in combat or traumatic violence might gain this goal. I am not sure how well world gen battles account for combat usable interactions...
ACQUIRE_WEALTH would be primary goal for any spell that allows for direct creation of valuable goods. It should be a secondary goal for spells that allow exertion of mental influence. This goal should be gained by historical figures that go through changing fortunes (rags to riches or riches to rage to riches, etc.). World gen needs to account for material wealth accumulation better...
STAY_ALIVE would be a primary goal for any spell that grants defensive abilities...
RULE_THE_WORLD a primary goal for spells that allow for mass creation of minions and for mind control. A secondary goal for spells that increase combat abilities and allow mental influence.
For more ideas, I think the Sphere brainstorming thread is good...
http://www.bay12forums.com/smf/index.php?topic=173189.0 just think of how each spell idea listed might have goals driving it.