Basically, Atlantis/Valhalla/Underwild/Dunes (and future) portals could get a small update on appearances rates for summoning after completing them.
For example, current rates for Dunes:
Classic: Rare 19.6, Epic 11.9, Legendary 0.9
Season: Rare 51.4, Epic 14.6, Legendary 0.6
Featured: 1.0
After completing the season (clearing ALL stages), rates could change to:
Classic: Rare 13.1, Epic 7.1, Legendary 0.5
Season: Rare 56.4, Epic 19.6, Legendary 1.3
Featured: 2.0
And even after completing the season in Hard, another update could happen:
Classic: Rare 7.3, Epic 2.4, Legendary 0.3
Season: Rare 61.4, Epic 22.6, Legendary 2.0
Featured: 4.0
Numbers are just an example.