Eternal Floette is still my pie in the sky, but more realistically probably some random gen 8 pokemon. Maybe just a cute mascot type, maybe something with connection to Zeraora, maybe just some rando that's not really important but gets screentime because it's really new.
Barring that I could see...Suicune. Suicune had increased importance due to Crystal, Crystal relreased this year and they could play up the very-slight suicune imagry Johto seasons toyed with. Actually now that I've written that out, Celebi would make sense too.