I really don't think Brock or Misty need to return as they have no relevance at all in XY or ORAS. They pretty much have appeared in some shape or form in every series so far anyways, so I would like a break for a while from seeing either of them again.XY:
-Brock & Misty would return again for a few episodes. They tell Ash that they are officially dating, and it makes him feel happy for them, but also kind of like the third wheel of the group. Also, after thet leave, Ash calls them on occasion. Sometimes individually or together.
I agree with this, it would be great to see either as long as she gets more screen time and is more active.-Serena would be a Coordinator, have a career we've not yet seen, or be a Connoisseur.