According to the just released Final Fantasy XV Ultimania guide Cor, Aranea and Iris are confirmed for playable characters in the upcoming Companions and Comrades DLC.
Looking at the season pass for Final Fantasy XV, the Companions and Comarades DLC will be the last DLC on the list.
It is also noted that the development team may include everyones favorite lady Lunafreya as a playable character.
With all of these playable characters it will remain a mystery why Square-Enix did not include them as playable characters from the beginning. Memories of switching out characters in older Final Fantasy titles remain fond.
If you wish to pick up the Japan exclusive book, you can do so here. Just note it has more pages than your average bible.