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Odds are soft resets and focus on making new fans (This is boring "Ash resets" tier)
Evens focus on building upon features and making good games (This is good "Ash experiences permanent development" tier)
Gen I established the core game
Gen II was a direct sequel that greatly expanded upon all the systems and mechanics while introducing tons of new ones, there are plenty of new evolutions to old Pokémon
Gen III dropped various Gen II mechanics, went back 3 years to Gen I, remade Gen I, and tried to be a soft reboot until they were forced to add old Pokémon, there were hardly any new forms or evolutions of old Pokémon.
Gen IV referenced all the games and brought back basically all features while fleshing them out further, tons of new evolutions to old Pokémon
Gen V tried to once again be a soft reboot with only new Pokémon pre-League, no evolutions/forms to old Pokémon, and some spiritual successors to Gen I Pokémon lines (Sawk and Throh to Hitmonlee and Hitmonchan, the Roggenrola line to the Geodude line, the Timburr line to the Machop line, etc.)
Gen VI focused on building mechanics (Super Training, PSS, Fairy-type) and new stuff for old Pokémon (Sylveon, Mega Evolution) but had troubled development
Gen VII is an outlier because it's an anniversary so there's references to all previous games, but it tries to reference Gen I the most and treats the player like a beginner. Gen I remakes again. This is also the darkest age of Game Freak.
Gen VIII continued to be the darkest age but was more balanced with fanservice than Gen VII, and Legends at least became Game Freak trying to target core Pokémon fans. So now DLC and Legends represent even Gens and initial versions represent odd Gens.
