Considering the additions of Fairy type and MegaEvolutions I have made a deduction. Why include just any old random pokemon in the Kalos Regional dex? Clearly its highly likely there will be a reason for each individual line included in the pre-E4 dex, meaning:
>Linoone new evo and/or Mega form
>Eevee already confirmed to have at least 1 new evo
>Azumarill included for Fairy addition with possibility of Mega form too.
>Houndoom included for evo and/or mega form
>Pikachu/Raichu included for possible fairy retype and/or mega form
>Octillery included for new evo and/or mega form

Those are the lines included in Serebii's confirmed regional dex atm.
>mfw MegaOctillery or Octillery evo