I'd say only Cloud, Ryu and Mega Man are truly viable out of those, probably Yoshi, Falcon and Lucario as well at least if your definition of viable is more lenient (it's hard to say because not every character has similar representation). Mii Brawler's source of reference is too tiny to say anything, while ROB, DK, Ike and Bowser get wrecked way too hard in some matchups to do anything without considerable help from a secondary. Wario was considered a high tier early on, but then he faded into completely obscurity barring Japan
where seemingly any character can top 8 a national.
The term "viable" gets thrown around too freely, probably partially because it lacks any context. Viable in what? Locals, regionals, nationals, supermajors? There are probably less than 15 characters who can win a supermajor barring a godlike player plowing through with an obscure character and partially riding on the matchup inexperience of their opponents. Not to take too many dreams away from Mewtwo players, but Abadango at Pound 2016 probably fits this picture to an extent.
Getting top 8 at regionals is way easier in comparison, and probably half the cast or more can do this. Yet, is there an aspiring competitive player with the fire burning in them whose ultimate aim is a top 8 at a regional? A viable character should be one that can win, not almost win but then die.
Actually viable is probably more like:
Large pool of "who the hell knows":
The main difference is that the characters in the upper category have actually shown stuff whereas the lower ones haven't, and most of them probably never will considering they've already had pretty long for that (1.1.5 or just sudden attention to a specific sleeper character could still potentially create something though).
and in case anyone is wondering, mega man can't be ignored because kamemushi
Villager and Pikachu had a lot of hype at one point due to ESAM and Ranai doing very well, while Bayonetta is a ??? but probably still at least decent, maybe even viable. The likes of Peach, Greninja and Corrin have a lot to show before they should even be considered in my opinion, meanwhile
Emblem Lord
has explained well why the Pits aren't that good, and to be fair, someone could probably whip one up for most of these.
Considering Ness' hype and success and then gradual but clear decline, it paints a good picture of how hard it is to actually win large things even in Smash 4. If I had to pick which character of the lower character pool had the strongest case to be in the upper one, I'd say aeiou
Regardless, there are 6 characters who can
barely be called heavy, unless you really want to make a clear distinction between superheavy and heavy in which case it becomes a bit more plausible. Perhaps that's decent, but the "fatty" design of being an actual heavy is completely missing and seems to be pretty much doomed.
I think this isn't a pessimistic view but a realistic one. Interesting to see what happens though.
Also,
Ike's recovery is ****ing terrible and it ruins the character