User blog comment:FairyRaven/Fairy Tail Chapter 328: Zodiac, Review and Commentary/@comment-112.205.41.120-20130421090106/@comment-15267891-20130421110615

Maybe Igneel, Grandine and the other dragons are in another dimension (or maybe another parallel world?).

Also, maybe Igneel is now a part of Natsu's soul. Just like another personality that became inactive and needs to be awakened. it's just latent. Same goes for the other dragon slayers. these are just some of my many theories. i just remembered Brain who has another personality (Zero).