Pokémon Sun and Moon - Legend of Alola says they’re our only hope can defeat Team Rainbow Rocket

I wouldn't exactly call it filler as it did have relevance to the plot and it comes up in later arcs. In Gen 4 Brandon The Pyramid King makes a reappearance, Paul's brother was a frontier fighter and that plays in to Paul's motivation to be a strong fighter, or something close to that it's been some time since I seen Sinnoh.
But regardless of Filler or not, it was post game content that happened in the main show (not a spin off), so it can happen again.