I would be curious in how the cosmology of power and magic would work in DF worlds. Like, we know there are deities that could offer power, but what about other sources of power?
So basically Deities of spheres with their powers are the predominant forces in the world, then there are lesser beings such as demons, fiends and perhaps some good counterpart to them. And then on the outskirts of the power spectrum could be magical forgotten beasts.
That is something I wondered about as well. Even the deities and demons seem to be linked, seeing as the deities use their powers to allow demons to access the mortal world in vaults (if I remember correctly). It could be something like each of the spheres have their own plane of power and deities are strong connections between the planes, while magical beasts have some weaker natural connection across planes. There could be myths related to this connection, like the dragons were the first children of the deity of the volcano sphere, like demigods, who may not understand their divine parentage but still retain some vestiges of their power. Sentient creatures with weak natural connections will have a high magical aptitude, and there could be a tiny chance for them to have a stronger connection and be very powerful.
As far as other sources of power, I always imagined the world itself being home to some source of power, and this is power is what elves tap into for their woodshaping and all that. Similarly, the world might have ley lines of sorts where magic power can be concentrated, and those with magical aptitude can access it easier. The mage's guild might have interest in investigating or setting up shop in these areas.
Some advanced interactions between items or materials could combine to have magical effects too. An enchanted silver and obsidian ring could give the wearer increased morale, while a enchanted golden ring with opals could give enhanced ability to persuade others. This feels a little bit too cliched and gamey though.
There could be magic based off of the spirit world or the spirits/souls of living and deceased creatures. It could tie in to macabre/fell moods and ghosts and the like. I would imagine something like this would be a very dark and taboo form of magic, requiring things like killing a sentient creature and hijacking its soul for a short time to explore an area or manifesting that soul as a ghost in order to frighten someone into giving in to your demands or distract guards or something like that, or even the old trope of stealing the life from another creature to extend your own.
There could be things like astronomical events that convey powers for a short time, alignment of the stars and all that. I don't know if the systems to support something like that are in, but it could just be handwaved away without really needing an actual astronomy model (though that would be neat). It would be cool if you had to fear something like an ancient and powerful necromancer who spent hundreds of years scouring the world gathering artifacts and studying old tomes. While amassing this great power, he discovers that every 263 years, the world is overflowing with magic for those who can access it. He has calculated that the next event will occur in less than five years, and sets out to recruit lesser necromancers under his banner in a bid to conquer the world. It also just so happens that you have established your fortress about this time...
There's a lot that could be done with the above scenario and the potential for something like that is really exciting. Suppose that necromancer was traveling incognito and stopped at your tavern on his way to visit another necromancer, and one of your wizards feels his power and warns the fortress. You could try to confront him right there, but it could go very bad very quickly if you're not prepared. Conversely, if you don't handle it early, you will find yourself fighting off a neverending army of the dead and desperately trying to form an alliance with the humans and the elves to fight off the thread.