Dr. Simon van Gelder ("Dagger of the Mind") could appear in a follow-up investigation story of what "Dr. Adams" was doing at the Tantalus Colony.
I think it would be interesting to see Starfleet's flag officers more in-depth. Maybe we could see Commodore Stone ("Court Martial") or Commodore Mendez ("The Menagerie") return. Both were very interesting guest characters. I always wanted to see Kirk receive top-secret orders in person from Admiral Komack ("Amok Time") in some Cold War-style drama.
Perhaps the most compelling guest character in TOS was Captain Pike. It would be absolutely fascinating to see prequel stories of Pike commanding the pre-"Where No Man..." Enterprise, handing it off to Kirk. It would also be fascinating to see more "Cage" era stories with the "Cage" crew still in place, Pike, younger Spock, and Number One included. I would love to see that era fleshed out more.
It would also be great to see what happened to Number One after Talos. Did she continue her career in the Star Service? Did she eventually command a starship or space station of her own? Would it be great to see a Commodore Hudec (Majel Roddenberry's maiden name) in command of a K-class starbase out on the frontier, working with Captain Kirk and other starships to solve a problem?
I would also love to see Yeoman Colt's character developed more. Maybe she went on to become a command master chief on some assignment like Cestus III, would up getting caught in a battle or some other disruption, and had to take charge and put out fires before the Enterprise could rescue her crew. It would be great to show her looking around the TOS-era Enterprise, recalling her days aboard, with Dr. McCoy or Yeoman Rand or, maybe ironically, Nurse Chapel.
One pseudo-character from TOS that TAS revisited was the Guardian of Forever. During the DS9 years, I once wondered if it could be possible for Sisko to visit the "City on the Edge of Forever", looking much as it did in TOS, and traveling through time to avert a recent disaster involving the Borg and/or Dominion.
Another compelling character, seen in only one ep, was T'Pau of Vulcan. There's an under-utilized gold mine!
I also mentioned other possibilities upthread. For some perverse reason, probably no good, I would love to see Proconsul Claudius Marcus assassinated.
