Banjo-Kazooie
I don't see why people would be giving Banjo-Kazooie 0% for chance, especially if they think that Steve has a chance. Ridley got in on the basis of fan demand, and obviously so did some of the DLC characters (Bayonetta explicitly, but it's highly probable Cloud's inclusion was related to ballot performance as well). Banjo-Kazooie seems to be one of the top characters wanted in the US, and while doing worse in Japan, is also a fairly popular choice there. The argument for Banjo-Kazooie over Steve is not at all complicated: B-K is more popular among Smash fans.
Anyway, the Vergeben leak, while not 100% confirmed, suggests that Microsoft has already granted permission for some content - specifically a Minecraft stage. While this might be limited to Minecraft - which gives Steve a chance, although it's plausible that a Minecraft stage would be the only content - it would totally make sense for Nintendo to prefer Banjo-Kazooie as a fighter. They would certainly be the more fan-service-y choice.
We also know that the head of the Xbox division would be on board with the idea. While other people in Microsoft have a say, I would expect the head of the their gaming division to be the most influential person on the topic. I wouldn't expect, say, Bill Gates or Satya Nadella to have much of an opinion on this and they're probably the only people who have more power to block this. Which isn't to say that other people in Microsoft couldn't cause a problem, or that Phil Spencer (I keep on wanting to call him Phil Spector) might not change his mind or desire more concessions than Nintendo is willing to give.
But the biggest questions are: does Sakurai want them? And will Microsoft consent to their inclusion? The latter seems to be only a minor obstacle, and if Vergeben is indeed legit (which it seems likely to be), it shouldn't be much of a problem.
As to whether Sakurai would want them... Aside from their popularity, I would say that they would be easy to adapt to Smash. They fit in the game aesthetically and making a moveset is no problem. They might not be the most unique fighters as there isn't an obvious gimmick for them, but the fan demand for them is such that he might be ok with that. Not every newcomer of Smash 4 and SSBU has a gimmick like the KO Punch or Luma or whatever. I don't know if he has any personal opinion on them, so I can't consider that.
A reason not to include them is that coordinating with Microsoft/Rare on them might be difficult (although it's my understanding that few members of the B-K team are still at Rare) due to time zone differences. And another is their lack of ongoing relevance. Of course, Smash might create an opportunity to change that...
B-K chances: 35%
They have a pretty good chance, but there's a strong possibility that that while Minecraft gets a stage, there is no 3rd-party fighter owned by Microsoft. Also possible that Steve gets in instead, or that the Vergeben leaker was just lucky or some of his information was legit but not all of it, or it was out of date and some of their plans changed, etc. etc.
B-K want: 80%
B-K was a great game. Aside from the fact that I like the characters, this would also make me optimistic for the possibility of Banjo-Kazooie and Banjo-Tooie getting ports or remasters on Switch (also great would be the possibility of other Rare games *cough* Goldeneye *cough* getting out of limbo). I never did play Banjo-Tooie, unfortunately.
Andy
A lot of what I said about Chibi-Robo applies here. The Wars/Advance Wars series seems pretty dead. It hasn't gotten a new entry in a decade (2008). That's less recently than Chibi-Robo, even. But it has historically significantly more sales and installments, so it is more notable overall.
On the other hand, he has a less obvious moveset. One possibility would be something like Pac-Man and his use of fruit + ghosts for attacks. Andy could summon miniature military units (probably retro sprites, as in the Infantry and Tanks AT) for use in attacks, since otherwise it seems difficult to incorporate aspects of the game into his moveset. But even so, since he has a relatively realistic human design (fairly normal proportions) this might not work as well aesthetically as it does with the more bizarre design of Pac-Man. This could provide him with some unique moveset possibilities, but he still requires more thought.
Ultimately, though, the problem here is that Wars is a dead series and Andy isn't a character with significant fan demand. While he could catch Sakurai's fancy, there isn't much reason for his inclusion. Wars does have an AT, but I feel like that's a pretty good spot for the series (future installments might look to the series for a retro rep, or the series might see a revival, although Fire Emblem seems to the sole focus for tactical RPG titles at this time).
Andy chances: 3%
Andy want: 40%
Meeeh.