Skull Kid's going to be one of the characters. There's no point in contesting that -- he's the mascot of the game. He wears the titular Majora's Mask, he's an Assist Trophy in the new Smash Bros., and the limited edition MM 3D comes with a Skull Kid figure. He's the one character people most associate with Majora's Mask.
I still feel really confident that the second character is going to be Tingle. Like Skull Kid, he's well-known and marketable -- there's a Tingle spinoff series, there's a Tingle plush doll, he's also an Assist Trophy in Smash Bros., and so on. He's goofy and annoying and "unpopular in the West," but he's been a recurring presence in the series for over a decade. It's also worth keeping in mind that the DLC is sold individually in Japan, and Tingle is pretty popular over there. More to the point, Tingle's map-plotting ability would be a really unique way of condensing Majora's Mask into a single character. He could spawn Clock Tower by drawing a map of Clock Town, he could summon the Turtle in a big swell of water by drawing a map of Great Bay, and so on. That doesn't even account for his balloon or ambiguous "fairy magic."
I'm having a hard time picturing Happy Mask Salesman as a playable character. Sure, he's integral to the plot of Majora's Mask, but he rarely shows up and doesn't do a whole lot. He's more of an Agitha-type character, who I consider to be an outlier (the staff really likes her). I could see Happy Mask Salesman showing up in a sequel or something, but I don't think he has a high enough profile to warrant his own purchase as DLC.
Fierce Deity Link will probably be one of the three DLC costumes, but I don't think a taller Link with sword beams (whose presence in Majora's Mask is optional) justifies its own character slot.
As an aside, I was bored and plugged a bunch of Majora's Mask characters into that Akinator Web Genie game, and Tingle and Skull Kid came out with a lot more results than Happy Mask Salesman or Fierce Deity Link.