For me, the biggest point towards Banjo is King K. Rool's bombastic return. He has one of the most faithful movesets in Smash and received 3 music tracks. The Donkey Kong franchise was treated incredibly well in general. King K. Rool arguably made the biggest splash out of everyone, something I'm sure didn't go unnoticed by Nintendo or Sakurai.
The reason this is important, is because Sakurai had to familiarize himself with Rare's DK games to make K. Rool's return as powerful as it was. Chances are that Sakurai looked at other Rare properties while he was at it. We can't know for sure, but it's not a farfetched assumption. Sakurai is now more familiar with golden age Rare than he was when he made Smash 4.
So yeah, King K. Rool's inclusion is the biggest beacon of hope for me. Forcing the Brave codename and stats upon Banjo is a waste of energy. You could also force those stats upon Cuphead if you tried. I only want to consider things that are actually tangible, like K. Rool, the cut Rare spirits, Sakurai's comments regarding the ballot, and the current collaboration between Nintendo and Microsoft.