User blog comment:CNBA3/Gray Spin Off series!/@comment-173.79.102.70-20140428073351/@comment-7418318-20140428083943

In addition to this, we didn't even see all the missions that the team went on. We see Lucy complain at least twice about how wrong a job went due to Natsu, Gray and Erza destroying everything, without seeing even a single second of said mission.

In addition there are times when Natsu and Lucy went off on their own (like in Fateful Encounter, even if it isn't fully canon), freeing Gray up to do something by himself.

Then a story could also focus on his past, before he joined the team or even so far back as to be about his journey from his home country to Fiore and how he ended up in Fairy Tail.