If you check message boards about Hyrule Warriors and ask what people expect, which characters will be chosen for the Majora's Mask Pack, you will get very similar results in most places. Pretty much everyone agrees on Skull Kid and there's not much arguing about it. However, the second character is where opinions really differ.
I see "Tingle" A LOT, usually with the argument that it was his first game. But I don't get it. I'm not a Tingle hater, just check some older posts about him, and I'm certain that he will be a fighter in Hyrule Warriors at some point. But including him via the Majora's Mask pack would be like including Ganondorf via some Ocarina of Time pack, just because that was his first game. Tingle is a recurring character, in fact he has the most appearances after Link and Zelda, and he should be treated as such. He should get his own Hyrule Warriors incarnation, which should be saved for a sequel, where adding more joke characters next to Agitha would be more acceptable.
I also read Happy Mask Salesman or Kafei a lot, but I have a hard time imagining a good moveset for them. They already admitted that they had lots of trouble turning Agitha into a fighter, so they will probably go for characters with actual fighting potential first. Mikau and the Deku Princess are very solid choices for that, but I feel like those characters should be introduced properly in a sequel, where they get tied into the story via the Legend Mode. The interaction with the others is what makes them really shine and come to life in this game.
The Majora's Mask Pack won't offer new scenarios or any other expansions to the Legend Mode. So, my bet is still on Fierce Deity Link. Not a costume or a weapon, but as a full character. Because together with Skull Kid and maybe Clocktown (and/or the Moon) as a new battlefield they would create a great story context without having new cutscenes or similar. In fact the whole "Fierce Deity vs. Majora" conflict never really got explained in Majora's Mask. It's just something that exists at the end of the game and since then has tickled the imagination of fans. It's one of the most mysterious parts of the Zelda lore, which is why it's so fascinating and spawned lots of fan fiction. People are excited by the unknown.
And this they can use. Having Skull Kid (using Majora's Mask as a weapon) and Fierce Deity Link battle it off in a new series of Adventure levels just works on its own without forcing a greater story context, which explains everything. There's no need for explaining, it's a platform for imagination. Actually the Adventure Mode always has been. I've seen Zelda fans, who like to fantasize about how the Adventure Battles, which can be rather random, came to life with their own little stories. And Koei Tecmo can ignite more of that with the "Fierce Deity vs. Majora" theme.
It's a meaningful and worthwhile addition, which works without a drawn-out story, and certainly a lot better than just randomly adding two characters without any real context.
So, I'm still thinking that Skull Kid and Fierce Deity Link will be the characters. Also, don't forget that both of them have First 4 Figurines. If they add Fierce Deity Link and Skull Kid, all Zelda First 4 Figurines will be playable in Hyrule Warriors!