Matt Eberflus did a fantastic job in 2018. He turned around a defense that was one of the worst in football in almost every category to an above average defense. No one saw it coming. But now that it happened, the improvements should continue to happen.
A big reason why the defense should continue to get better is Darius Leonard. The rookie should only get better with more time in the NFL. He will get bigger, faster, and stronger. Leonard will be the leader of this defense for many years to come.
The large amount of cap space the Colts have will also help the defense.