Bandana Dee conversation has driven me to lock in my roster predictions. I'll try to characterize my confidence in each with % chance.
Newcomers I think are in:
K Rool: I feel like it's likely he won the ballot. I don't consider him 'realizable' for Smash 4 DLC at all due to having no recent character models or animations that could be used as a base. Plus as many have pointed out he had considerable proportion issues similar to Ridley. There is no way they would try to get him done in a reasonable time period for DLC with these issues even if he got 1st. His brand new model for Mii costume is evidence that Nintendo is aware of his popularity. I also believe DKC6 is in development and will feature a return of Kremlings, making him relevant again. My reasons for this are the street signs in Odyssey, the reference in TF, and the Mii costume. Plus with TF port to Switch done by Retro themselves I doubt Retro is done with DKC.
Confidence: 40% (cons: growing obscure, has a much worse size problem than Ridley ever did, moveset potential almost exclusively from Captain, Baron, and Boxer incarnations)
Bandana Dee: It may not seem like it, but I actually do believe the datamine thing points to his inclusion. I do think it's likely he'll be the resident joke character this time around. Don't be surprised if he sucks on purpose (I hope not), but even if viable I expect him to be characterized by looking like he's out of his league and knows it. Think Luigi's expression in dash attack.
Confidence: 35% (cons: basic enemy)
Isaac: I know this sounds like wishful thinking, but I find it incredibly strange that Isaac lost his AT but gained music. He was a very popular and iconic AT. I believe Sakurai has a "if time allows" list and he put Isaac on it. I don't have any evidence other than a hunch, but it still makes sense to me.
Confidence: 25% (cons: I'd call him semi-obscure, relevance issue, another sword user even though he really doesn't even need to use it to have full moveset)
Rhythm Heaven Rep: There's some small evidence someone from RH was considered before and got cut. The Gematsu leak ballooned RH support. I think RH has a solid chance.
Confidence: 10%
On 'If time allows' list:
I consider some of these more likely to be in game than the characters above. Basically, I think their chances of being in planned roster are high, but so are their chances of getting cut do to time.
Elma: I feel very confident she's on the planned roster, but I also am convinced she is the very first cut if they run short on time. I think Rex and Pyra have a high chance of being DLC regardless of Elma's inclusion and I think Sakurai could foresee this and instruct his team to make her last. Ironically, I consider her chances of being in higher than K Rool simply because I think she's a lock for roster plan, I just think she has a super high chance of being cut.
Chances of making it: 50%
Pokemon: Pokemon has so many reps that I just don't see them as being a first priority, especially when gaining 3 cut pokemon vets back. 4 from the perspective of people who didn't get any Smash 4 DLC.
Chance of making it: 30%
Dixie: TF gives her a huge boost, but I honestly think K Rool has a better chance due to popularity in the Smash community and this game giving the impression of Smash community targeted fan service. I still think she'll be on planned roster, but the second most likely to get cut for time.
Chance of making it: 30%
Saki: I have felt since his AT debuted that Sakurai wanted him in but couldn't justify it, which is why he is an AT. He's a very notable missing AT (unless someone spotted him and I missed it). I think Sakurai might finally put him in.
Chance of making it: 10%
Lip: Lip is bizarrely popular considering her circumstances. I'm sure Nintendo has noticed. She has the perfect opportunity to be the baffling inclusion in this game. She is so obscure I don't even think most Nintendo fans have heard of her, yet she is popular in the bubble that is Smash fans.
Chance of making it: 5%
Echoes:
Shadow, Dark Samus, Ninten: I feel like Shadow and Dark Samus are at least 75% chance of happening each. Ninten 50%. I don't see Mother coming back in a rebooted Smash and I think that's what we're getting next and I think Sakurai knows this and is sending Mother out with a bang.
Already Confirmed:
Ridley & Inkling: obviously they are both announced already, but I was fairly confident in both of them being in before they were officially announced (yes, I was confident in Ridley despite 'too big'. He was never known for being big but rather intimidating and scary in Metroid and I had confidence Sakurai would realize that).
And yeah, I expect no third parties at all and I don't think the Simon leak was real.