I'm currently GMing a game of Fate Core for a group of friends. The group has a range of experience with tabletop RPGs, including two total newbies. I have the most experience if the group, including around a decade of experience GMing for other systems.
We're all completely new to Fate, but so far are enjoying the system. I have read through the a Fate manual pretty thoroughly a few times, and other resources (blogs, YouTube, this StackExchange, etc) to prepare for this game. We just played the first session (well second, really: we did world-building and character-building previously, but ran out of time before we could play) and while most of it is going smoothly, I'm trying to figure out how to structure an extra for one of the characters.
We're playing in a setting in the not-too-distant future of our solar system. It ended up a little bit cyberpunk, a little bit Firefly, and a little bit Dune. Thanks to that last influence, there are various semi-mystical groups in the system with various mind-powers. One of the players has chosen to play a character with powers such as these. The idea is closest to Bene Gesserit training: mind-over-body and suggestion are the core of the trope.
Extras are one of the parts of Fate I understand least. They have the least amount of explanation in the book, and it mostly seems to be handwaved as "...and more!" This is clearly something that affects the narrative, so it should have an aspect permission, which his character already has. It involves actions, so should use skills and stunts, but here's where I start to get lost.
I don't really want to create a new skill just for mental powers. Existing skills that fit the bill are Lore and Will. Will fits better thematically, as the concept is more Zen meditation/yogic powers than psychic wizard, but I'm worried about making Will too important, since it already has value by providing mental stress boxes, and being the likely candidate for defense against intrusive mental powers from antagonists.
Stunts seem a good fit for specific mechanical use of powers. Things we have discussed are using the mental skill for fight (limited use or FP cost?), being able to "swap" skill values temporarily (at a cost), and being able to create temporary aspects on NPCs via suggestion. But in terms of cost, I'm really lost. Should access to these powers be a stunt? Should each power be a stunt? Should it cost refresh to take? Should it cost a Fate point to use? How often should it be usable?
This first session we ran it as "Fate point to use Will instead of Fight for one round" but that seemed too prohibitive. We also had "swap two skills levels. The benefit lasts through this scene, the negative lasts through the next scene" but it was never used. I've considered making the Fight one attack-but-not-defense, or possibly being able to use it for either, but only one at a time.
I'm really struggling to make this interesting without being overpowered (something the player also voiced concern about), but I'm still so new to the system that I don't have a good understanding of what the consequences of these changes would be.
So, RPGSE, can you help me out? How can I model subtle mental powers as an extra without ruining the balance with the other players?