I don't see Nate appearing at all. Rosa and Hugh are up in the air. If they do appear do they have to be rivals? There's no league right? So can't they just be involved with Team Plasma's plot and want to save their home region of Unova? Do they have to be generic rivals? Aren't there enough rivals as it is? If they do bring in Hugh and Rosa I hope it's like Lyra, where it's not a rivalry, just there to be showcased and have them involved with Team Plasma. But personally I don't think either are really needed. The writers have enough side characters/rivals this gen to last until gen 6. Use them.
Parena or Serecham both equal Serena+Pancham! Sereevee or Serevee equals Serena+Eevee!