I think he would be a mistake for him to come back. I'm one of IT's biggest fans from his time here and he changed the trajectory of our franchise when he came and I will always be grateful for those fun moments he provided to us - the 50 point game, the game where he played a day after his sister's death, etc. He was a driving force behind our success.
BUT were he to return he would have to accept a different role from what he used to play for us, in a time when he is playing for a max contract. Anyone who thinks he would be willing to come off the bench as a super sixth man and defer to Kyrie, who was responsible for him losing his spot in the first place, after years as the top dog in the same team, doesn't understand the concept of pride in sports. I think it would be a major source of tension in the locker room and would totally ruin the chemistry and dynamic that the Celtics have built.
IT found it hard enough to defer to LeBron in Cleveland and LeBron is an all time great who has been with Cleveland for years - can you imagine him deferring to Kyrie, the upstart who took his spot as The Man in Boston? They are both ball dominant players and I don't think we have room for both players.
IT will flourish in a situation similar to what he was in in Boston before - as the alpha dog in the team. Bringing him back into our situation now is a team culture and team fit disaster waiting to happen. This would only work in NBA2K where the players you trade back and forth don't have actual feelings.