Follow-up project on a paper in submission I submitted to a journal a fairly novel psychology theory in the form of a manuscript that our research group put together. I am the first author and did most of the research. The paper is currently with a journal. I see the first reviewer has returned his report. We are waiting for reviewer two.

My next follow-up project would be to create a type of questionnaire to measure the theory. My key question: Should I start working on such a project before receiving comments from the peer reviewers on my initial project/submission? Couldn't the peer reviewers/editor ask for major revisions which would affect my questionnaire?

My PI has continuously been asking me for the project status but I've been telling him: I'm waiting to first hear back for comments from the "main psychology theory" before building a questionnaire. Is this the correct way of doing things in academia - my approach?
I think you should never delay your projects based on unsure events. The referee report may take a long time, and you'll have wasted precious time. Also, it's unlikely that a referee will change your mind about your own work. You probably know what the questionaire should look like by now.