The top eight trainers in the World Coronation Series so far only have the first, second, and seventh-ranked trainers identified (as of 3/9/21): Leon, Lance, and Raihan. Possible identities for the others include the other Champions.

In fact, a somewhat 'cheap' listing could be listing them in generation order as the first and seventh gens have 'the player/rival' as the champion, thus making it

1: Leon2: Lance3: Steven4: Cynthia5: Alder6: Diantha7: Raihan8: ?

(Due to their statuses Gary and Iris probably do not fit in the list)

While fans will fiercely debate how right or wrong Steven ranking above Cynthia is it, of course, not the only way to list them. It's just the laziest way to make a theme out of it.

The position of eighth could easily be another Galarian character, possibly Piers given how close he was in skill to Raihan in-game and maximizing the current-gen advertisement. Wallace as a Contest-Champion who was implied to have lost the title to Steven could also be a candidate.

A Frontier Brain like Palmer, Brandon, or Anabel could also work.

The fact that Melony has never lost a battle against Raihan and he's in the Masters's Eight, she could be a candidate.