The basic idea is to not make them feel like regular magic, usualy providing somewhat less powerful effects or completly different ones but with different mechanisms.
Wild Gifts are powers related to nature, the wilds, animals, and so on. It does not mean it is only nice, nature can be a bitch

Gifts use a new power source "equilibrium" which represents your balance against nature. 0 is good, the higher it is the worst it is.
There is no maximun equilibrium (ok there is, 100000...).
Each talent use increases equilibrium and it decreases *REAL* slow (consider it does not).
When an actor tries to use a wild gift, it computes a chance of success based on curent equilibrium and the cost of the talent. Obviously the higher the equilibrium the higher the chances of failure.
In case of failure 1/10th of the cost is still added to equilibrium pool and the turn is lost.
However there are benefits too:
- gifts are not affected by fatigue, as such they can be used as well in full plate (if you can wear it) than in a robe
- gifts are not magic, as such they are not affected by antimagic, spell reflection/absorbtion, and so on
Slime powers are the first wild gifts but more to come.
I have some fun ideas for antimagic, it will be a wild gift school. A talent in it could allow to create a personnal antimagic shield which absorbs spells cast at you and increases your equilibrium based on the cost of the spell, until you fail your equilibrium test and the shield comes crashing down.
Ideas welcome !