1) The first letter of the codenames chosen for the DLC fighters could be based on their first letter. So P for Paku and Piranha and J for Jack and Joker. B could be for Brave and Banjo. It would explain why he would fit with the term Brave.
2) His stats are virtually perfect.
- Brave's slide almost mimics Banjo's slide from his own game
- His size would work because Banjo himself is not that large of a character. If people worry about him looking too small, Kazooie occasionally popping up from his backpack would visually make him look bigger than the stat may seem. Banjo is the hitbox so Kazooie can do whatever the Hell she wants.
- Floaty jump makes sense for Kazooie on his back. Plus Banjo's initial jump is just naturally floaty in the Banjo duology.
- No wall jump fits.
3) The Rareware spirits supposedly in the code.
4) The popular Phil Spencer quote saying he wouldn't mind adding them in.
Now since it's a theory and since the lack of correlation to Banjo and Brave is still a large issue, it's not much to bet on. I wouldn't be too optimistic right now. Banjo is now just another name for the pile, along with Erdrick, Sora, Bravely Default, and Geno.