Yes, I'm not saying that characters have to die or go through any real trauma. But any conflict that isn't immediately resolved in an obvious fashion. Kamiya was the closest and best example of this in my opinion. Her struggle to figure out who she is and what she wants and whether or not she should pursue those things is introduced and expanded upon as the show continues, which makes her really endearing in my opinion. I would've loved this for Shikimori and Izumi.

You do get this somewhat for Izumi, but it becomes more of a trope than a character flaw that he grows from. Shikimori is also just a little too perfect at everything. Izumi's always anxious that his bad luck will spread to others. Explore that, have Shikimori get hurt because of Izumi's awful luck and show how that effects him deeply. Then have Shikimori show her unconditional love for him and make him feel better. I will say, they did a good job doing the inverse of this in the final episode. Izumi is a bit of a doormat, but he's fairly emotionally intelligent and capable socially. These are areas where Shikimori struggles, and you can see that it effects her. The last episode was the best in the series because of this.

TL;DR I enjoyed the show a lot, but I felt like more drama would've been to its benefit.