rereading nikolai's and zoya's duology and "i like to keep you on your toes, nazyalensky. anxiety does wonders for the complexion." is still one my favorite nikolai lines in the entire saga it's going on my bio
Summary: Zoya and Nikolai have lost touch over the years, but they reconnect with each other (and some old friends) when they and their partners move back to Os Alta to raise children. Life before this had seemed like it was calm, predictable, sometimes even dull. But adulthood, marriage, and parenthood were never bound to be simple for these two.
Things Kaz canonically does in the two years between Crooked Kingdom and Rule of Wolves:
(presumably) massively expands the Dregs' territory
buys Pekka's old club (the Emerald Palace) and completely renovates it, turning it into The Silver Six
massively expands the Crow Club to the point where it's "three times the size of every other establishment on the block"
builds an underground tunnel that goes from the Crow Club to the Geldstraat, where the Van Eck Mansion is
takes Jesper out on jobs with him often enough that Wylan has jokingly banned Jesper from answering the door when he knocks
learns about Ketterdam's Suli laborers and picks up additional knowledge of Suli culture
keeps up with Inej's whereabouts and helps her take out slavers
expands his information network to the Kerch colonies
is on friendly enough terms with the King of Ravka that he taught Nikolai how to pick locks and Nikolai feels comfortable personally writing him a letter when he needs to steal the titanium from Kerch
disguises himself just to follow people around on the streets
was planning to steal the titanium from the military base anyway just for fun
And that's just the stuff we see from Nikolai's and Zoya's incredibly limited perspectives during their Ketterdam sidequest
I 100% agree with Zoya when she thinks that "maybe Kaz was like Nikolai, a boy with an unquiet mind, a man in perpetual need of challenge" because ROW makes it so obvious that Kaz is bored and incredibly restless in his success. Someone get our boy a new life's purpose and a subscription to a long-running unsolved mystery podcast stat