oni link is evil...
and in majora's mask, who said that on link didnt take over?
i mean, u can ONLY use him for boss fights (without codes i mean), and the battle against majora looks like a personal fight between the oni and majora himself...
in the comic, majora says to link:
lets play tag, u are the "oni" since in japanese, the one who tags, is called "oni", so its kinda of a word game...
anyway, he is a demon god, thus evil... who only lets link control him because he wants to destroy majora... thats my interpretation at least...
After some research: I change my case; FDL is evil.
At first, Majora could not destory or defeat Link, so this time, he tries to corrupt him with an evil mask, but again, fails. He tries to corrupt Link's mind so he can use Link to feed off the despair with him.
Link can remain pure of heart and wield the mask and not succumb to the evil mask's temptations. By not giving into its temptations, Link uses the mask for good, to destroy Majora.
What this also means is that Link cannot use the full power of the mask.
This is because he does not want to participate in evil, AND he has a pure heart.
The god can obviously grant powers to Link because he is sealed away in the mask.
But being said the god is evil, he is displeased and will not let Link use corrupt powers unless he is corrupt.
I don't think the god can take over Link's mind, only temptations and the soul are evident.
Also, being said that Link is using the mask for good instead of evil (destroying Majora), the god is probably not introducing the corrupt powers to Link unless he is using the mask for corrupt reasons.
The final smash should be, like, something that is corrupt.
But the final smash should not involve anything with Majora's mask. (literally the mask)