Currently, the only names allowed in merge fields in subject lines are First, last, and nickname, but we prefer to use the Primary Salutation field as the typical addressee field. Because some people prefer to go by another name than their first name, or their first name is an initial, we align our system to use the preferred addressee name in the primary salutation field when using merge fields in email, and it would be great to use this in subject lines, too, so they will at least match up.