Reviewing: potential conflict from knowledge of other researcher's activities I do methodological research in Field A and am aware of the the related methodologies of Field B but have not used them. I was asked to review an article from Field B, which I initially declined due to lack of expertise. The editor asked me to reconsider because the article makes some claims about Field A and he wanted comments specifically from that perspective. I agreed to review.

I am reviewing the article and the editor was right to be concerned. The paper is otherwise excellent, however the authors are claiming that what they are doing in Field B is actually Field A (and using terminology from Field A but have ignored substantial literature). I am fine to write the review. However, I would really like to make a comment recommending that any mention of Field A should be removed from the paper (it would be fine without it) and noting that the overlap is an interesting question in its own right that should not be dealt with from the perspective of just one of the fields within an empirical paper about a specific topic.

I actually think that a specific methodological paper about the overlap and differences would be very worthwhile, coauthored from both fields and preferably written as the outcome of some sort of discussion at a workshop where each field trained the other. While the review is blind, I strongly suspect that the authors are from the institution that would be the most appropriate to develop such a workshop and paper. However, I am already aware of a researcher in my field who is drafting a paper (from the perspective of Field A, with no involvement of authors from Field B).

If I make the comment about interesting question in its own right, am I suggesting the separate paper? If so, is that a problem given that I know someone is working on something similar? Can I suggest to the person working on it that perhaps he should involve authors from the other field? Can I suggest a workshop to both the person working on it and the authors of the paper I am reviewing?
Your job is to do the reviewer, which means 1) evaluating the quality and 2) helping the authors improving the paper. Half of the things below are unrelated to this. You even got a specific task from the editor. Limit yourself to that. 


  If I make the comment about interesting question in its own right, am
  I suggesting the separate paper?


This is exactly the only thing you should do IMO: "make a comment recommending that any mention of Field A should be removed from the paper and noting that the overlap is an interesting question in its own right that should not be dealt with from the perspective of just one of the fields within an empirical paper about a specific topic." 


  If so, is that a problem given that I know someone is working on
  something similar? Can I suggest to the person working on it that
  perhaps he should involve authors from the other field? Can I suggest
  a workshop to both the person working on it and the authors of the
  paper I am reviewing?


This to me seems not OK because potential conflict of interest and because you are using confidential information for purposes that are beyond the review. I get the impression that you also risk of revealing yourself which you should never do, if it's a blind review. Wait for the paper to be accepted and available online and then you can contact all the relevant people pretending you had this idea reading the paper and without saying you are the reviewer. If the paper is rejected then drop the idea of the workshop or contact the editor and agree with him/her on a proper way to contact the authors.