Frankly speaking, I don't really think the character after ARMS has to be a HUGE reveal or a BIG-FAN FAVORITE and if it's not people aren't going to buy the pass. First of all, if that were the case; wouldn't it make more sense if they made the first character a HUGE reveal? That way more people would've bought the pass immediately rather then waiting on it. Also just because the last 2 E3's gave us Ridley and Banjo doesn't mean we are getting another big fan favorite like Geno or Waluigi, that just seems to be looking for patterns that are barely there (although I do think the character will end up being big in some way as E3 is when they reveal more iconic characters). Remember that both Ultimate and FP 1 started with BIG reveals like Inkling and Joker, but then while people were expecting an equally big reveal to start off FP 2, we ended up getting ARMS. Besides, we have little information on how this pass will even go. It may end up as entirely 1st parties for all we know.
I definitely think the character will be "big", but a "big" reveal to certain people is different compared to a "big" reveal for others. For example, some characters like Walugi/Geno/Rayman/Crash being fan-favorites in the West, but on the other hand you have Rex/Reimu/Saber/Arle being bigger requests in Japan, and that's not even considering other "smaller" regions with different fan-favorites.
Last time, we had reveals like DQ to appeal to the JP audience and BK to appeal to the Western audience. Unless we somehow end up getting 2 more reveals right after ARMS, which I doubt; appealing to both audience would be difficult to pull off. Sure we could end up getting Crash or Geno next which would be HUGE reveals for the Western fanbase, but in Japan they would be more of smaller but still "cool" picks. Likewise, we could end up getting Rex or Arle as the "big" E3 character, which would be seen as "good" picks in the West, but absolutely HUGE for Japan.
There are a couple universally popular characters that could appeal to both Japan and the West AND the Smash fanbase. I think the frontunner in that case would probably be Sora, as he's huge everywhere, is big in the Smash community, and is considered an "Impossible" pick. Slightly smaller characters could be something like Dante or 2B, who share most of Sora's strengths to a lesser extent (though they aren't seen as "impossible" picks). And then we get to characters like Sephiroth who are once again popular enough in both regions, but are less desired within the community itself.
TL;DR: People should keep there expectations in check. The E3 character can be anyone from the likes of Sora to the likes of Rex. There's no real reason to default to Geno or Waluigi, although they could still be the fighter.
EDIT: Also Max Brass or Min-Min please daddy Sakurai.