Julian was introduced as a metahuman expert but in reality he didn't have to deal with them too much. He did in the first half when we are getting introduced to the character but later in the season the focus was Savitar and he got moved to the back. This is probably part of why he didn't return for a season 4. But, this would be the perfect time for his character to shine. I am of the honest opinion that had his presence this season would have been great and boosted their storytelling capabilities. He is a metahuman expert and this season is about having lots of new metahumans created. And what do you now, we're two thirds into the season and we've still not come across them all so he would still be active and an important part. Also, if he was here he could have taken more of a leadership role and Iris could have been out as a journalist. This makes sense as he already was in charge of the CSI department so a leadership role in the team isn't crazy. But also, they could have had Iris shown to actively be trying to find these meta's instead of us being told about it. Also, it would have provided an outlet to show the police forces reaction to Barry's imprisonment whilst he was there. I just find it ironic he decided not to do another season when this could have helped himself and been perfect for his character.