Masked Man
Popularity: Among Earthbound/Mother fans he has decent popularity. However, in Japan Kumatora seems significantly more popular. Among most people who aren't hardcore Smash fans, he's unknown. He also doesn't exactly top the Smash fan demand list.
Relevancy: He plays a very significant role in Mother 3.
Design: He does appear to use a weapon so that already would change him up quite a bit from Lucas, so he should probably be fairly unique. There are IMO some other, more deserving characters like Isaac and Takamaru that would mix swordsmanship with magic (PSI) or other projectiles though.
Roster and Competition: Earthbound is already the lowest selling series with multiple representatives. Like, by a wide margin. Many series deserve their first (Punch-Out, Golden Sun, Xenoblade, Takamaru, Mii), second (Metroid, F-Zero, Kid Icarus, even Pikmin, Yoshi or Wario) or third rep (DK, Fire Emblem) before Earthbound gets a third, IMO. Combine that with the potential for the big series (Mario, Pokemon, Zelda) to get new reps and 3rd parties (Pac-Man or otherwise) and I don't see room for all of them, much less another Earthbound character. Sakurai would have to love love love the Mother series to give them another rep given all the characters he'd be cutting in line, imo. He had even considered replacing Ness with Lucas, rather than simply adding Lucas.
Not to mention there are other characters that could get in, such as Kumatora, Porky, Jeff or other playable characters. Porky I consider unlikely even among Earthbound reps because he doesn't ever fight outside of a large machine. But Kumatora is more popular in Japan and would add a female character. She does, however, have the same moves as Lucas considering Lucas stole them from her. So she'd either be a third Ness clone, or require them to de-clone Lucas with some of his actual moves. Jeff would definitely have his own moves though. Masked Man might be important enough and have fewer move set complications and therefore be considered the most likely Earthbound addition... but that isn't saying much.
The fact that Earthbound does have two reps is some evidence that Sakurai likes the series a lot. But enough to give them three reps? I dunno about that.
Masked Man chances: 0.9%
More than I would give to the other Earthbound characters. After thinking about the number of worthy reps out there and the limited room for new characters (definitely less than 15 imo), I probably should've downgraded some of my previous scores even further, and I'm one who tries to give more realistic (i.e. lower) scores in the first place. Anyway, I'd be quite surprised at there being another Earthbound character, so I should give him a score that implies that I'd be quite surprised. This is less likely than being dealt three-of-a-kind in five card poker but more likely than a straight.
Masked Man want: 43%
I don't hate him, but there are a number of other characters I want first. I haven't played Mother 3 yet though, so maybe that would change my opinion. I do love Earthbound, but I'd rather see Donkey Kong get K Rool and Dixie, for example. If it weren't for the limited space (i.e. we were getting a 50+ roster) I'd want him to join I think.
Vaati prediction: 10%
I'm gonna go higher than I think, since apparently even Simon Belmont can muster almost 10% (I love him, but he should've rated closer to 1%). Fact is we know there is maybe one slot for a Zelda newcomer (whether Toon Link stays or not) and Vaati has some stiff competition for that slot, if it exists.
Nominations:
5x Mike Jones
