From what I have seen and heard over the past several weeks, I will try my best to compile everything that seems to help the case of Banjo-Kazooie to be in smash.
-Sakurai has mentioned he wanted to have B-K in Melee but couldn't (Rare was bought out by Microsoft)
-This year marks B-K's 20th anniversary
-Phil Spenser publically said he would like to have them in smash during the ballot
-From past comments on here, and posts elsewhere, it appears B-K were always in the top 5 of most voted. Sakurai has said that he is using the smash ballot votes for newcomers this time
-Minecraft is already on the Switch, which is a Microsoft IP. This isn't directly related to better chances of B-K, but it is indirectly related since both are Microsoft owned
-Going along with the last point, Vergeben as a part of his 'leak' (his sources haven't always been reliable but have been so far for smash, but he hasn't been given 100% info for this game) said that Minecraft in some form will be in Smash. Again, not to say that means B-K is attached to it, but they seem to be the most likely candidate of Microsoft if they got involved with the game
- In smash 4, they released several mii costumes. Inkling, King K. Rool and Chrom (confirmed), Isabelle, skull kid, geno (suspected), and a bear costume. This one could be a stretch since there are lots of other mii costumes deconfirmed and bear mii doesn't exactly equal to Banjo, but hey you never know
-Speculation is sometime in the near future Nintendo will release an N64 mini just like the NES and SNES minis. Rare had many top sellers on N64, so you would think their games would be on a N64 mini, assuming Nintendo makes N64 minis. This argument seems to be the most complicated, but if Nintendo were able to strike a deal for Rare games, that would look super good for B-K in smash
These points could be meaningless overall, hopefully not. It's just we have never seen a stronger case for Banjo-Kazooie to be in smash ever and it seems the duo has a really good chance, from an optimistic point of view lol.