The other theory of Link's future is that he becomes the Feirce Deity. The Feirce Deity mask is a strong form that appears to have Links appearance. The markings on his face is a symbology of war paint signifyingthat he is always battle ready. How would Link become the Fierce Deity one would ask. He would have to reach a higher state of being and train consistantly. The Shade theory of Link is another highly popular theory but take time into keeping an open mind.
In Majora's Mask the Feirce Deity's mask could be aglimps of Links future self, the eventual ascension to a higher power. This is only speculation as one of the many paths Link could take.
One possiblilty to support this theory is the effectiveness adult Link wheilds a two handed sword such as the Giants Knife and the Big Goron's sword especially during the fight with Ganon. When Link has the Master Sword flung from his grasp the two handed sword is an option to battle when considered that you bought the Giants Knife (even when broken)or did the Big Gorons quest to obtain the Big Goron sword.
The Feirce Deity could have power manipulate time and sent a mask containing a connection to the power he wheilds to Termina which whould set it in young Link's path giving a possible opening to ensure the Feirce Deity could exist. Such a feat is possible considering the multiple movies and series containing when a future self sends impotant message to a past self to ensure a correct path is tooken that would help or prevent a path that would lead to destruction.