Enchantment magic to mind control characters. For instance if you used it you would get a strong hook and it would be permanent or you could add events to make them do stuff and such like that.

Already planed :)

Illusion magic spell to make two NPCs rivals of one another.
Blood magic spell to change to someone else's dynasty
Conjuration magic to summon demon advisers
Conjuration magic to summon a familiar, giving the mage a bonus, depending on what kind of familiar it was (Wolf familiar giving prowess, Owl giving learning, et cetera)
Life/Earth magic spell to increase fertility of the province that the mage is in (bonus tax? for x number of years)

I don't think teleporting is possible, is it? Moving your army from one province immediately to another province? Maybe Time Magic would be able to increase army speed by 1000% for one month, at high cost? I think one of the Commander perks lets that commander move quicker, maybe the spell could apply a custom Commander perk temporarily to that mage, and then remove it after the month.
A limitation of PDX games that I do know, is that if you're moving province by province, no matter how much you improve movement speed, you can only go a maximum of one province per day, thus the "for one month" thing.

I'm just spitballing here. I might add more, if I think of good ones.

Hey I saw you removed the death tree because of the UI limitations in the change notes. Also I see some good suggestions here for mage lifestyles it may just be easier to just organize the perk trees in groups of 2 or 3 in different lifestyles rather then trying to expand the UI if you plan on adding more. Like Mage/Wizard and Necromancer for the darker ones.

i'm working on making the 3 trees i have good first, if i can't find a ui solution by the end of the week i will just use more lifestyles but i'd rather not. It would require changing lifestyles more often and i'd rather people be able to stay on just 1, plus im fairly sure too many lifestyles will have the same issue.

The mass celerity perk gives a small static bonus to army speed already, i'm looking into making a "teleport spell" but it seems kind of op though

if teleporting had a long cast time and gave a debuff for a minute after coming out the other end then that could help. the larger the army the more difficult it should be.

i dont think teleport is possible, if it is i have no idea how to do it

I don't know if this is possible (or already included because for the moment I don't play ck3 anymore) but it would be nice if the mage (player and AI included) could only become a master of a certain number of magic ( maybe only 2 or 3 max) and not all.

I'm planning on adding enough trees that you cant unlock all of them without cheating but i could add a game rule to limit the number of trees you can master, or maybe slow down xp gain based on number of already aquired peks/ filled trees

Perhaps buff the lava golem? Or nerf the stone golem, I guess. Lava golem has double the stats of the stone golem but gives you thirty times fewer soldiers per cast, meaning they're way less impactful in combat than stone golems. It feels somewhat disappointing to finish out the tree and get a spell you'll never use.

Ill take a look, but it is intended mainly as a siege weapon, its siege stats are so high it should basicly solo castles, i must admit though i havnt really had time to look at balancing much yet

I think the Golems are too OP. Get ten of each can defeat anyone, they can't even die out because of it. And there is no fun after that.

yeah im not happy with how summoning works, i'm looking in to ways of restricting the number of each you can have but i haven't figured out a good way of doing it

make them become Men-at-arms maybe? Or massive increase the cooldown, and increase the number as well.

I dont know if this is in the scope of the mod... (its not) But I got to thinking about the necromancy tree, and figured I'd share the thoughts. Would Necromancy be more like a Dark Lord religion? You'd have to do something terrible to your land / peoples Tyranny or whatnot to get an event maybe that switched you. But you could handle all of the "summoning" rather then through spells, through specialized religious units. This religion would obviously be evil to just about everyone, but finding correlations and customizations might be pretty fun to do.