I want to see him improve on his timing passes more than anything. He seems to hold on to the ball pretty long before unloading.
I think that has a lot to do with his approach towards the game - not releasing a pass until he feels reasonably confident that it'll be "safe."
Those timing passes generally entail throwing a quick strike to a spot on the field where your WR isn't, but you must anticipate and trust that your WR will make it to that spot and make a play. It's definitely a pass of faith.
I think the reason why we see Russel Wilson holding on the ball longer than is expected, and why the pass seems late is that he throws it once the WR has already made his cut and got a little separation from the defender, rather than throw it to a blank spot on the field and then when the WR makes his cut the ball is already right there.
Considering that Russel Wilson has only had one season to establish chemistry with the receivers, and his concern over turnovers, I really expect him to improve on this for next season.