If Juventus wants him he will come back as a free agent.
I thinks if Douglas Costa joins it will make the Juventus club stronger to be able to win the title again with Douglas Costa and what is clear is that Douglas Costa has enough experience and he has previously joined Juventus for a long time. I think Douglas still has a close relationship between the players until now Douglas Costa is up to the point where he wants to return, but currently Juventus has a strong squad and seeing that Douglas Costa is already 33 years old, I think if he returns to Juventus, I am a little doubtful that he will be able to play full time at the Juventus club, but there is nothing wrong with if that Juventus need his services again considering that he has contribution to the club before.