To be fair, this team was unwatchable the first half of the season, and deserved all the criticism they received.
As for improving this team in the offseason, Jaylen is the key. He is an elite shooter, but struggles offensively when given too many responsibilities. It looks like the game doesn't come naturally to him, and he is thinking too much on the offensive end. It would seem to me, Jaylen should mirror his offensive game around the likes of Klay Thompson, Ray Allen, and Reggie Miller. Move without the ball, use screens, and catch and shoot. This strategy seems more instinctual for his skillset.
Jaylen does get his points at the end of the day, but they don't seem to come in crunch time. To me, having two legitimate scoring threats to close a game will improve this team leaps and bounds. Right now, Tatum is the closer, with Smart being the #2 option, since Jaylen essentially becomes invisible.