Arms Character
Chance: 99% - There's no way Spring Man misses this iteration of Smash in my mind. Arms got promotion and support from Nintendo only rivaled by Splatoon, and we saw how that turned out. The moveset basically speaks for itself, there's the possibility for a unique Arms-switching mechanic, and we knew about the game right in the sweetspot for Sakurai to have planned a character for it here. If Spring Man isn't base roster, he's undoubtedly DLC. I honestly think he might even bring a Ribbon Girl alt/clone with him. I don't really think any other characters have a chance. These two are the ones on the boxart after all.
Want: 55% - I'm indifferent to Arms. I tried a demo for it at an event, and it was fun but it didn't blow me away, and I never ended up buying the game. If Spring Man makes it, that's just another check mark on the list of most likely additions. If he doesn't, I'd certainly be confused, but I wouldn't care too much. A friend doesn't believe me that he's a lock though, so I have an extra 5% for being able to gloat when he does get in.
Rhythm Heaven Character
Chance: 90% - Y'all are gonna think I'm nuts for this. But I think a Rhythm Heaven character is a lock this go around. Gematsu was undoubtedly real in my mind, but from an earlier point in development. It helps that the only other part of Gematsu aside from the Chorus Kids that didn't come true, Chrom, we know was in development at one point thanks to an interview with Sakurai. So even then, just because they were cut, doesn't mean that they're a lock for the next one though, right?
I think the important part here is how far along they got before being cut. Ice Climbers we know had a ton of development done on them, to the point that they were working on the Wii U version but the 3DS version stopped them from happening. Because they got so far into development before being cut, there were still plenty of leftovers from the Ice Climbers' development presence in Smash 4: A Smash Run enemy, an unused series logo, and a few songs.
The character we know was planned to be in the running was the Chorus Kids. As a trio of characters, they undoubtedly ran into the same 3DS issues as the Ice Climbers, and, I would be willing to bet money that they got cut at a similar phase of development to the Ice Climbers. After all, what they left behind was: A Smash Run enemy, an unused series logo, and a few songs. I basically see these little guys about as likely as the Ice Climbers, because they undoubtedly got just as far before being scrapped. Ice Climbers only get the edge for being veterans that had a previously-made moveset, whereas the Chorus Trio likely could have not had a complete moveset yet. Considering the insane similarities between the Chorus Kids and the Ice Climbers though, I am very confident they got to the same level of development.
Want: 70% - I'm terrible at rhythm games, and Rhythm Heaven is no exception. I'm even terrible at spelling the word rhythm. But I'm supremely confident in the information I posited above, and it'd be so satisfying to be right. Plus, my girlfriend loves the franchise and she'd be ecstatic to see it get some rep in Smash. However, I'm not as big of a fan of it being the Chorus Kids specifically that are likely. I'd much prefer Marshal, who's basically a chorus kid that's an actual character, the franchise's mascot, and he wouldn't have a complicated trio mechanic going on with him. Game and Watch-style moveset summoning characters from all the minigames please!
Predictions
Daisy: 14.02% - A certain faction is almost definitely going to skew her chance rating, but they'll be too much in the minority to bring her up too much.
Paper Mario: 23.58% - He's in a better, but not great place either. A lot of people are rightfully going to cite him being a third Mario, along with the nigh-universal panning of his sub-series' most recent titles. Relevancy and moveset potential will likely bolster that a bit though.
Nominations
Holy cow, I was right on the money with Chrom. I've never seen a prediction that close, dang.
Dark Matter Blade x 10 please. Gonna get that guy on the schedule first, then comes Scorpion.