While Sabrina was interesting with her psychic powers and all, she is suppose to be the Gym Leader of Saffron City and can't really travel around that much. Besides, as CyberCubed already mentioned, we hardly ever see any of the Gym Leaders Ash defeats after he gets a badge from them.
I'm almost kind of surprised that you mentioned Blue, a character from the Adventures manga series who has not been in the anime series. If you really want to see more into the background of those characters, then I suggest reading the Adventure manga could work for you. I know that Viz will start re-releasing the series starting from volume one this summer, in case you're interested.
I'm not sure if I would call Casey a rival since she only had like half of the badges needed for the Johto League by the time that Ash was on his way there. I do agree with you on how if there were to be another Chronicles season, it would focus quite a bit on May, which would be awesome. I don't agree that they wouldn't have enough other characters to focus on for other episodes though. They still have other main characters that used to travel with Ash or have some importance to Ash or his frienRAB.
They could do something about Max and what he does with his parents while May is off traveling, maybe an episode about what happened with Brock when he saw his family again, maybe something about Misty handling being a Gym Leader, even though I thought that they did one too many before. There's also Prof. Oak, Tracey, Ash's mom and, of course, Team Rocket. The series has a good amount of characters who aren't in the main traveling group that they could focus on for another batch of Chronicles episodes if they really wanted to.
I'm not sure what kind of side characters they could focus on since I don't think that they would focus on Casey or Richie again, considering they haven't been mentioned since their Chronicles episodes, but they definitely have enough characters to focus on for a few episodes.