Imagine if Pokémon transfered from the Gameboy VC games have a special ability (like a second hidden ability) or just a new flag (like the XY pentagon) that retains all of their original properties

>Magnemite/Magneton
>type retained on evolution allowing for pure Electric Magnezone
>Clefairy/Clefable are pure Normal
>Jigglypuff/Wigglytuff are pure Normal
>Mr. Mime is pure Psychic
>type retained on breeding allowing more Magnemites and Normal-type Cleffa and Igglybuff and Psychic-type Mime Jr.

>they act like they are operating under the old type chart
>their Bug attacks are super effective against Poison and vice versa
>their Ice attacks deal neutral damage to Fire Pokemon
>Psychics immune to Ghost
>since Dark, Steel and Fairy didn't exist, all of these Pokemon will act like the type doesn't exist, dealing and taking neutral damage.

Other changes from Gen I would be too much, like the Special stat, the physical-special split, and numerous mechanic changes. But I can see these things maybe happening.