With such a strong forward line, we can probably allow them more space and play a +1` in defence almost continuously. Typically this is going to be one of BJ, Hepp or Zerret. BJ is best at this role imo.
Personally I hope Zerret spends more time in attack in 2018. He was unreal in the last game in 2017, got 30+ disposals, kicked 4 goals and scored 3 Brownlow points. If Zerret kicks 30+ goals it will propel him into the top 5 players in the AFL.
Saad is a very good defender. Can lock down and run off, but just needs to improve his disposal at the end of his runs. The problem Saad as a running half back is if he runs too far, the opposition defenders have time to man up the forward options. However, Conor has elite disposal and is the running defender who should get last use of the ball approaching the arc.