The more I think about it, the more something along the lines of Gengar actually makes sense to me
Let's say Sakurai bases his newcomer picks entirely on the ballot. That he wants this to be his "Ultimate" Smash Brothers game, a celebration of the series as a whole, so he focuses on giving the fans EXACTLY what they want. It's not even that far fetched, the 3 newcomers we've gotten are extremely popular, fan demand has been cited multiple times in interviews and blog updates, Sakurai himself said he brought back every veteran because "He believes that's what the fans want."
We have no idea what the ballot results were apart from Bayonetta winning overall, disregarding "Unrealistic choices". Polls have tried to get an idea, but those are never going to even touch the massive casual crowd that would have voted on the ballot. But we can make a few educated guesses;
Pokemon is unbelievably popular, more so then pretty much every other franchise on the roster. It makes sense that a ton of votes would have gone towards people's favorite Pokemon. Hell, I voted for Sceptile myself!
Now say you're Sakurai, and you want to pick a Pokemon newcomer. This time, you want to pick the one that your fans want the most. What might those results look like?
Well, first off, it's going to be a Generation 1 Pokemon. Sceptile had a decent run, but that was mainly in the core Smash fanbase, the ones who wanted a grass starter to nicely fill out the trio. I wouldn't be surprised if the Pokemon did ok on the ballot, but it can't compete with the juggernaut that is the Gen 1 fanbase.
So who'd top that list from Generation 1? Call me crazy, but I think
would have scored the highest.
The starters,
in particular, would be pretty high, but its got to compete with
, whose already made it back.
I could see characters like
,
, and
, but they're already disconfirmed as Pokeballs.
One's like
,
, and
(Praise be) are really popular, but they're not exactly built to be a Smash fighter, I don't see people voting for them...
doesn't have hands, which is a damn shame because that would be awesome.
In my opinion, the major contenders that are left are
,
,
, and
. Gengar fits that bill best. There's very little overlap with other characters on the roster, no weird complications to make them fit in, it's one of the most expressive Pokemon in general, and it's got some serious potential for a weird, interesting moveset. It could poison fighters, confuse them by messing with the players inputs, put them to sleep and eat their dreams!
And sure, there's Game Freak, but Sakurai has stated that they go to the Pokemon company to
research potential fighters and who might be popular in the near future. Game Freak doesn't pick for him, he still goes his same criteria as usual:
"Well first of all, we talk with the Pokemon company. What’s the hot Pokemon? What Pokemon are in the movies right now? And really do a lot of research on that front.
For example, X and Y are coming out – of course, we haven’t done any market research because they’re not out yet, but we look at the animated series or movies and anything like that and again, find out which ones are going to be central to any of conversations in Pokemon going forward.
But it’s not just that – going back to just what we talked about, what’s unique about them? Where do they fit in with the rest of everything else? What do they have? It’s a combination of those things. "
The way he words it makes it sounds like he goes to them simply to find characters to add to his potential roster of Pokemon. And it's not like Gengar isn't a "hot Pokemon" right now.
TL'DR, Gengar actually kinda makes sense if you keep the desires of the casual audience in mind.