I have to say this up front. One thing I didn't care for in TNG was returning characters particularly in the form of Llwaxana Troi. That coupled with characters going back to Earth or Risa whenever they felt like it. It made the galaxy seem small and the ship didn't feel like it was on the frontier.
TOS only did it once with Harry Mudd and it looked like a total coincidence. And I wasn't crazy about the episode he returned either. Having fresh characters helped lend the feeling they weren't hanging around the neighbourhood. And characters weren't going off to Starbase or Earth for leave whenever they felt like it. It's those little things that added to that sense of being "out there."
To that end I hope STC sticks with that sensibility.
Sadly TAS did revisit a number of previous characters and I wish they hadn't. In quite a few cases they were the weaker episodes.