Hi! Your field is interesting, and I am looking forward to take some introductory courses on it in the near future.

While waiting for responses from experts in this thread, I suggest discussing this important question with your systematic review (SR) co-investigators as well since SR writing, by convention, is a team sport. Ideally, your SR team will include at least one SR methodologist (i.e., one who has extensive experience and knowledge in SR writing specifically on neuroimaging) and at least one domain expert. Narrative reviews and taking note of "trends" in how outcomes are dealt with in relevant neuroimaging studies will also be help to guide you in determining your primary and secondary/auxiliary outcomes.

I can also suggest the first two answers; you should do about the subject should be a literature review. The predictors and outcomes that focus on the most impactful studies on your subject should be included. If you want to do a meta-analysis, you should target quantitative data common to your generally included studies, which requires a literature review. I also find a very useful Cochrane youtube channel (https://www.youtube.com/channel/UCV85pBXQRwdv5LTgGt_Xt4g).

For a systematic review of the literature, I strongly recommend that you seek help from a librarian.