Publishing a non-invalidating comment to a recent paper- should I contact the original authors if I know them? This question is similar to this one: Publishing a comment or contacting the authors privately?
but has a few differences.

A colleague of mine has recently published a letter responding to another author's paper.  The letter agrees with the general points of the original paper (paper A), but offers specific additions that would improve the general idea from paper A.  After reading this letter (let's call it paper B), I think the additions have merit but should also take into account some new ideas which will build on both paper A and B. My comment would not invalidate paper B but it would argue that their suggestions should also take into account my own.  I know the authors of paper B personally and am in good relations with them.  I am acquaintances with the authors of paper A. 

My goal is to provide comments that improve this general idea (by introducing new ideas) and reference both papers A and B directly.  I have two questions. Question 1 is, should I:


I) Write my idea without contacting the authors of the other papers
and submit it to the journal 
II) Contact the authors of paper B,which my comment would most directly address, discuss my ideas, and suggest a collaborative paper on which I am first author
III)
Contact the authors of paper B, discuss my ideas and let them know I
intend to publish a comment (not to bring them in on the paper but to
let them know a comment will be submitted) 
IV) Take another course of
action


Question 2 is,  if I bring the authors from paper B on my letter, will it hurt the chance of it getting published as a comment to paper B if the authors of paper B are also on my letter?

I think the authors will probably agree with my suggestions.  At the moment I am leaning towards option II as I think option I is unprofessional, but I have never been in this situation before so I do not know what is professionally appropriate.
My comment would not invalidate paper B


So you are in a very different situation than the post you quote.

In your case you can manage so that your comment is not harmfull to the authors of B.

You clearly seem interested into getting a reward by publishing your idea (I do not say this in a negative way), so I would do something like III. I do not think it makes sense to add more authors on an 'opinion' paper.

Do you need some input from the authors of B or are you just trying to be polite? In the last case I would only contact them after having a first draft or at the same time you submit comment. You can turn your email into a 'thanks publisfor this inspiring paper' note. The authors should be happy that you are building upon their idea!