Okay, so we have had two characters in the works since before Mewtwo released that we know nothing about yet. We have a third character who's been worked on sometime within the last two months as evidenced by the inclusion of an extra character data slot following last month's update.
My speculation is two newcomers, possibly with unique mechanics and/or 3rd-parties (Inklings with an ink mechanic/Shovel Knight/King K. Rool), and one semi-clone vet/newcomer who doesn't require a lot of time to develop compared relatively (Wolf/Dixie Kong).
I can see Inklings, Shovel Knight, and Wolf being released (Shovel Knight deal would have to be made well in advance, Inklings for popularity and semi-promotional reasons especially so as time went on, and Wolf due to being an extremely popular veteran who wouldn't take that long to make comparatively), with indications that more characters are still coming without being specific as to who or when (K. Rool would have to be one for his popularity, and with how much Nintendo is promoting Yo-Kai Watch in Japan and NA I can easily see Jibanyan making it in as well due to being highly recognizable and especially popular from a popular game in Japan).
Jibanyan Vs. Pikachu, battle of the monster-capture mascots... Interesting.
Seems like giving even more attention to a direct competitor to one of Nintendo's top grossing franchises would be risky, but marketing theories have yet to pay off (and that's exactly what Nintendo is currently doing anyway) so that's not a valid point against him.
In the end, I think that Jibanyan would have to pass the 'Inkling' test. What I mean by this is if Inklings get in, they will set the precedent that very recent characters can get in provided their popularity survives the 'fad phase' and shows little doubt of losing fan favor over a long period of time. Right now, Yo-Kai's a fad. If it's still just as popular in six months
and a character as recent as Inklings make it in, then Jibanyan has a shot. Inklings also have to make it in for this line of reasoning to hold up.
Unfortunately, by that virtue Jibanyan faces another problem - the time it would take for this character to establish its staying power pushes it close to when I estimate to be the end of Sm4sh DLC, so it if takes longer to accomplish that goal he might miss the boat. If it happens, Jibanyan would likely be the last character we get.