This is my ideal roster, which hasn't changed all that much since the last time I posted it (whenever that was):
I won't bother making a prediction list, since aside from Inklings and most unique veterans, I have no idea what to expect.
For the sake of making it easier to tell who the characters are, I'll list them by series, and explain any choices that might be controversial. Also, Ice Climbers, Wolf, and Snake are back, because ideally, they wouldn't have been cut in the first place.
Mario: Captain Toad
Donkey Kong: Dixie Kong, King K. Rool
Metroid: Ridley
Zelda: Midna
Pokemon: Decidueye (he seems prominent enough, but I've been out of Pokemon for several years and I'm still trying to get through Pokemon Y)
Kirby: (Bandanna) Waddle Dee
Xenoblade: Elma (I haven't gotten the chance to play through Xenoblade Chronicles 2 yet - I'm still working through the original, and November's Switch releases pretty much drained me. Maybe Rex and Pyra would work better; I have no idea)
Joy Mech Fight: Sukapon (I just find it fitting that the main character of Nintendo's first fighting game makes it into their big fighting crossover)
Mysterious Murasame Castle: Takamaru
StarTropics: Mike Jones (I find it odd that the series hasn't even gotten a
sticker in Smash Bros. - apparently, it not getting released in Japan has something to do with that, but I think he deserves a chance)
Golden Sun: Isaac
Sin & Punishment: Isa (went with Isa over Saki mainly for the jetpack and possible tag-team gameplay, with Kachi joining in for specials)
Eternal Darkness: Alexandra Roivas (while Pious Augustus is more visually distinct, Alexandra is the main character, and I'm not sure how willing Nintendo would be to have a guy with a human spinal column for a staff interact with Mario)
The Wonderful 101: Wonder Red
Splatoon: Inkling
ARMS: Ribbon Girl (could be replaced with Spring Man; I haven't played ARMS yet for the same reason as Xenoblade Chronicles 2, but both of them seem equally prominent in the marketing)
Rayman: Rayman
DOOM: Doomguy
Shovel Knight: Shovel Knight
No More Heroes: Travis Touchdown (aside from Suda 51 consistently pushing for him, I figure that his luchadore moves and strange Dark Side powers (like turning into a tiger) would make him entertaining and far more than an average swordsman character)
EDIT: August 4, 2018
I'm going to make a prediction for Smash Ultimate's roster, now that we know more information about the game. I'll put it in spoiler tags, since it seems kind of unfair to claim it as my prediction when an overwhelming majority of the roster has already been revealed.
EDIT: November 25th, 2018
If I had to guess the five DLC characters, I'd say (in no particular order)
-Elma (Xenoblade Chronicles X) - the lack of new Xenoblade content in the base game seems weird to me, given how much the franchise has expanded
-Bandanna Waddle Dee (Kirby) - with Ridley, K. Rool, and Dark Samus added, Kirby and Zelda seem the most overdue for new characters, and most of the major options for Zelda were made into assist trophies instead
-Geno (Super Mario RPG) - apparently, his Mii costume is nowhere to be found, and Sakurai has gone on record saying that he's wanted to include him for two games now
-Heihachi (Tekken) - his Mii costume is also nowhere to be found, and unlike fellow fighting game character Akira, there's no sign of him anywhere, even as a spirit. Plus, Namco-Bandai is developing the game
-Banjo (Banjo-Kazooie) - this is something of a long shot, and Rayman or Travis Touchdown could easily take this spot. This is only if the datamining that showed
is true.
I'm working under the assumption that spirits don't confirm anything either way, and I could easily be wrong about all of this, as all of my other attempts at making a roster have shown.