Is it unethical to have a possible future coauthor to review my current paper? Many think it is unethical to have your coauthor to review the paper; many think it is not. 

In my situation, I am discussing possible collaboration opportunities with researcher A. He knows the field well and might be the fittest referee for my current paper, which is also in this field.

Is it unethical to nominate him as a reviewer?
Many think it is unethical to have your coauthor to review the paper; many think it is not.

Many journals and associations have strict guidelines on this matter; I suggest you to take a look at the answers to Conflict of interest as a referee, especially @DavidRicherby's one, and possibly also to other questions with the tag conflict-of-interest. In any case, a recent co-author, in my opinion, is a big no.

In my situation, I am discussing possible collaboration opportunities with researcher A. He knows the field well and might be the fittest referee for my current paper, which is also in this field.
Is it unethical to nominate him as a reviewer?

I assume these are serious discussions and not just a generic exchange of niceties &quot;this is interesting, we should discuss it some time&quot; at a conference. In this case, I suggest nominating him, but mentioning the possible conflict of interest at the same time. There is no harm in being explicit about it; just let the editor know and decide for themselves. If you have only a tiny text box to nominate possible referees, you can argue more in the cover letter.