One annoying dynamic that occurred in Iron Flame is almost guaranteed to continue in Onyx Storm, causing issues with the ongoing narrative. A particularly large plot point of Iron Flame was the newfound tension between Violet and Xaden regarding the lack of trust and honesty in their relationship. Those complications were mostly due to all the secrets Xaden kept during Fourth Wing. Their back and forth forced them to reveal more and more about themselves to each other, though that tension continued to escalate this issue for them.
https://screenrant.com/onyx-storm-continue-violet-xaden-annoying-dynamic-iron-flame/