Fearmongering about Xenoblade becoming the new Fire Emblem never gets old.
Still waiting for that Sakurai bias to kick in and give me a second character after 6 years. It literally has less characters and the same amount of music arrangements that Fire Emblem had when it debuted in Melee.
Nobody is "fearmongering" here, I just don't think a series is immediately owed another character because it decides to change its main cast every game. Like it or not, Fire Emblem is the most applicable parallel. I have no strong feelings one way or the other about whether or not Rex actually gets in, even though he personally doesn't excite me that much.
It's hyperbole, I don't genuinely believe Xenoblade will suddenly have 8 characters across the next two games. I wish you the best in getting a new character, I just don't think it NEEDS one. More music would have been nice though.
Also, this may sound dumb, but considering my MW is a first party who doesn't have their series represented on the roster at all, I think Xenoblade is pretty lucky to have gotten that opportunity in Smash 4. I would prefer to see more new series get that privilege as well.
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Also to seriously answer the question of who could be added in the next game... it's way too early to say, but it's fun to think about. One of the big reasons why we can't make too many assumptions though is because this game may see significant cuts. Or maybe we'll get Ultimate ported over and over again. I think these characters have a pretty good shot either way.
Waluigi - He's the new Ridley, except he's from an even bigger series and is wanted by a casual audience in addition to the hardcore fans. I doubt that will subside and doubt it will be ignored.
Tom Nook - Animal Crossing is massive and this guy is one of the biggest Nintendo characters still absent from Smash. While his potential to be distinct from Villager and Isabelle is debatable, I think there's more than enough to work with. He can use his tail to whack people and fly around, Timmy and Tommy can come in to assist with some attacks, he can swing around giant bell bags. Nothing to worry about here.
Octoling - Maybe an alt, maybe an echo, maybe a semiclone, maybe fully original with Splatoon 2 weapons. I dunno, all I do know is that I'd be surprised if they aren't playable in any capacity in the future. Splatoon is one of Nintendo's biggest modern successes and with Inkling here to stay there's plenty of room for additional content.
Skull Kid / Midna - Zelda is overdue for someone new, and I think by the next game they won't be able to ignore this any longer. Both of these characters are very beloved and well known despite their significance mostly being regulated to a single game. I think Skull Kid has a slight edge on account of Majora's cult status but I think both of them are great candidates.
. (not even buff their projectiles, but maybe give them a little more surviving power?) 
