I'm not entirely sure who Demise is, so I won't refer to that person in this post.
Moving on, if Ganondorf was to wear Majora's Mask, it would make him more destructive in my opinion. As we already know, Ganondorf is an evil being, he uses his current power to spread fear amongst the people and to acquire a high status, thus acquiring more power. Majora is a much more evil and sinister being though, as he has no motives for what he does, and he doesn't hesitate at the thought of what is basically genocide. This is proven when he uses Skull Kid though the Majora's Mask to cause the moon to fall onto Termina, thus destroying all life in the land. For what reason is this? No reason at all, other than causing chaos and devastation. Ganondorf actually needs the people to be alive, otherwise where would his power lie? It wouldn't lie anywhere, because he'd no longer rule over anyone nor could he use his power for sadistic reasons. If Ganondorf was to wear Majora's Mask, all motives for his plans would be gone and he would no longer care about the life of anyone. That is why I believe that Majora's Mask would cause Ganondorf to become more evil.
This is assuming that Majora's Mask was able to corrupt Ganondorf's mind though, instead of Ganondorf being able to resist it. If he could resist it, I doubt very much would happen at all because if he could harness Majora's power, he wouldn't use it for anything other reinforce the fact that he was the all mighty, all power leader of the land.