In an interview with Jake's Takes (via YouTube), Ralph Macchio was asked about the possibility of bringing in Jaden Smith and Jackie Chan into "Cobra Kai" and how they might mesh with the existing character on the show.
Macchio responded by noting that the show's creators, Josh Heald, Jon Hurwitz, and Hayden Schlossberg, refer to the show's universe as the "Miyagi-verse." He elaborated that any characters who interacted with Pat Morita's film character Mr. Miyagi are all fair game for the show. However, as the character did not appear in the 2010 film, which didn't seem to take place in the "Miyagi-verse," it seems to rule out any characters from the 2010 reboot appearing on "Cobra Kai."
However, Macchio worded his response very carefully, saying that the characters of Dre Parker and Mr. Han, played by Jaden Smith and Jackie Chan respectively, would not be able to appear on the show. This does suggest, however, that there's a possibility that Chan or Smith could appear on the show by playing entirely new characters. This would allow the show to nod to the reboot without breaking the logistics of the universe.
