I think both Hoenn and Unova have similarly contrasting environments to reuse Unova's environmental elements in R/S remakes. Putting Hoenn in B2/W2 would be nonsense imo. The main reason Kanto is connected to Johto is because they share League. Hoenn and Unova have their own Leagues, so what's the point of putting them in one game? And saying that Unova's characters and Pokemon will randomly move to Hoenn is even more nonsense.
To decrease chances of Hoenn being in B2/W2, think about Emerald's storyline - after Kyogre and Groudon escaped from Sootopolis, Rayquaza should stay there and battle you because it's game's mascot. But it escaped as well! I think it's proof that in R/S remakes, after they three escape, Kyogre or Groudon, depending on your version will reappear in Cave of Origin to battle. I think R/S remakes were already planned during Emerald's development and that's why game creators decided Rayquaza to escape, to make plot support for Kyogre or Groudon reappearance.