Rayman has a strong legacy of games and appeared on many Nintendo consoles. Ubisoft has probably already approached Nintendo about him. I think he's one of those characters that will be destined to appear in SSB sooner or later.
Banjo-Kazooie also has a legacy and lots of vocal fans. What they lack in quantity of games they make up for in how crucial they were for the N64 era. We know Phil Spencer tweeted about them, but the finer details of this kind of deal between Microsoft and Nintendo make it hard to really say.
Simon Belmont appears to be a strong contender for a second Konami character, seeing how generous Konami is being with their IPs this time around. The Vergeben leak also seems to point towards Simon's inclusion, however Vergeben is only maybe 70% reliable and he could still be making obvious guesses. Simon does seem possible and has been a popular request for a long time.
Sora might have the most popularity out of all these characters, however. I can't tell you how many times Sora has been mentioned by the SSB community. We know Square is cooperative, however Disney and their vile/backhanded business strategies might be enough to deter Sakurai from even considering an approach to the terrifying corporate giant (lest we see a sudden surge of Disney content that ruins our beloved SSB). That's really the only factor preventing Sora, which is a good enough reason for me. Seeing how Disney has tried to acquire Nintendo repeatedly and always been denied, there are definitely sour feelings there. So Sora has one very big obstacle while the stars are aligned everywhere else (in relevance, popularity, etc.)