Someday/theories

Maybe the "unfinished" ending of  "Someday" is ironic. Because when Itsuki found all the effects, he just continues playing. When the player goes to the window after he wakes up, it symbolizes the title itself - "Someday, the player will  get out of his room".

In the School World, you can find, in a certain classroom, a child version of Itsuki who is sitting in a chair alone, watching his playing classmates through a window. Itsuki was likely a solitary boy in the elementary because he was different than the other child, most specifically more intelligent than the others. This could explain why you receive the Nerd effect from him, and because of that intelligence the other child regret Itsuki and the only way to be accepted by the others is being like them. This could explain why a classmate of Itsuki sends you to the Monochrome effect location, as an attempt at assimilation. Itsuki would rather be alone than be a part of their groups, but has regrets of his introvert behavior in the Question World.

The NPC from which Itsuki receives the Lab Coat Effect (An adult with colourful wings wearing a lab coat) could be a figure Itsuki looks up to or could potentially be Itsuki himself in the future. Many of the effects and locations relate to science or technology (Wrench, Nerd, Lab Coat, and Clockworker effects, Testing Area, Space World and Hospital), so he is likely interested in science, medicine or engineering. Itsuki may want to become a scientist or researcher when he is older, and that NPC looks very similar to him.