Authorship for insightful suggestion during a talk?
You should at least acknowledge the person who made the suggestion.  

I would also ask this person if they would be interested in working with you to coauthor the paper.  If the person who made the suggestion is very senior or doesn't work in your area, then they'll likely be happy with the acknowledgement.  

However, a suggestion like this, without any further involvement in the writing of the paper isn't by itself sufficient to merit coauthorship under guidelines such as the ICMJE guidelines for authorship.