Hi, Gerudo here! Yes I purposefully used my actual Tumblr name. Anyways I have a headcanon.
Basically, it's about Termina. The headcanon goes that Termina isn't really...real? Well it is, but it's not...static, or linear, like the rest of Hyrule is (without Mask and/or Ocarina interfering). Termina is a dream dimension of sorts that manifests a world to torment whoever it bonds to, and it's currently bound to Mask, as it has been since he first fell into it. It bonds to whoever falls into it, one person at a time. That's why it doesn't work like the normal world, why the people there are so familiar, why Oni's mask looks like Mask but older and albino. It's why clocks are such a theme: Clocktown, the clocktower, the three-day counter; it's imprisoning the Hero of Time, why not drive all that trauma home. Termina is almost alive, a sort of semi-sentience, a bit more sentient than the Divine Beasts. But this is not a benevolent place. Termina steals the souls of people who enter, specifically the ones it bonds to. Others are just the icing on the cake. Those who fall, when their souls are taken, become actors in future manifestations, puppets doomed to be an aid in Termina's continued consumption.
Termina can steal a soul only when that person truly succumbs to despair. It is a patient construct, content to whittle down its victims until they fall. It captures them with strange wonder and tortures their psyche without their knowledge, until they finally fall and beg for everything to end.
Majora is a strange thing. Without Demise's curse, Majora would simply be the manifestation of Termina's intent,vin the form of Mask's little side-hobby, preying on him until he, too, fell. With Demise's curse, Majora changed, no longer Termina's will but with a semi-will of its own. Wishing to burn the world. A lucky thing that Termina is the only world Majora knows. A very bad thing it no longer is.
Mask is lucky, but there is no guarantee he will escape. Termina does not allow its victims to leave, but it knows how Mask does so, anticipated, took advantage of Mask's good intentions to create a vessel he could not bear to leave intact. With Majora, Skull Kid, the falling moon, and the friendly people of Clocktown, Termina keeps Mask coming back. And who says it has to let him leave when Majora and Demise are gone? No, better to let the void return its prize and keep him in its clutches.
Let the void burn; the void will soon return its prize, and the damage will heal. Termina...the place where people go to die. The terminal between peace...and eternal torment.
So yeah that's the basics. There's more in my head but I won't run this too much longer. Termina wants to steal Mask's soul. I wonder how the other Links would react to this, probably not great. Actually, probably pretty bad. Like really, if the Happy Mask Salesman told them when Mask wasn't around or something. Wait that's an entire new idea. Fanfic muse, my best friend and worst enemy.
Majora knows all this. Mask does not. And yes, everyone who's from Termina is actually not from Termina here but instead have been made into puppets after having their souls stolen. That's what Termina wants to do with Mask. The other Links are dressing on the turkey.
Now I must write something for this or it will not let me be.
Good luck! Seems like you've got it pretty fleshed-out already, so I bet you'll have a smooth go. Oh, and have fun with all that angst and horror potential