These are remakes or Ruby and Sapphire. Steven is the champ. I'm like 99% sure of it. And frankly I'd rather face him than Wallace when I've already faced Water earlier and if I see another Tentacool or Pelliper I'll scream at that point.
As for who I'd like to see make a cameo appearance...oh god the list is quite long. But I definitely wouldn't mind Crasher Wake, or maybe Wulfric in the Shoal Cave. But perhaps my top pick would be Korrina. Steven could have asked her for assistance in understanding the Mega Stones and Mega Evolution, since it is supposed to be her family's legacy or something to that effect. Heck, this could also be an excuse to have the Lucarionite handed to you in the game...