Gen I: Monochrome
Gen II: Partial color. Day-night cycles.
Gen III: Full color
Gen IV: Partial 3D.Day-night cycles.
Gen V: Partial 3D, fully animated sprites. Seasonal cycles.
Gen VI: Grid-based 2.5D
Gen VII: Hallway based 3D. Day-night cycles.
Presumably things will go down like this;
Gen 8: Hallway 3D, return of destrucable background.
Gen 9: Hallway 3D, return of seasons.
Gen 10: Semi-open 3D, proportionate models.
Gen 11: Semi-open 3D, full open-world encounters.
Gen 12: Open world.