At the release of PokeBank all the Pokémon Safaris will show a 4th Slot.
In this slot a previously unobtainable Pokémon will appear.
>Normal
Raticate
Persian
Porygon
Stantler
Lopunny
Glameow
Bouffalant
>Poison:
Koffing
>Grass:
Bayleef
Grovyle
Grotle
Servine
>Fire
Quilava
Combusken
Monferno
Pignite
Darumaka
>Water
Croconaw
Marshtomp
Feebas
Prinplup
Finneon
Dewott
>Ghost
Misdreavus
Yamask
>Psychic
Unown
Elgyem
>Electric
Tynamo
>Ground
Baltoy
>Bug
Kricketune
>Fairy
Cottonee

The Safaris like Steel, will get a previously obtainable Pokémon.