Michelle Trachtenberg spent three seasons as Buffy’s little sister/mystical key. Since that time, she has been working the big screen. Some movies we would like to forget, (cough) “Ice Princess.” She is also in the upcoming film “17 Again” starring Zac Efron and Matthew Perry. Now she has landed a pilot for Fox, which is already the home of Whedon characters River Tam (Summer Glau) and Faith Lahaine (Eliza Dushku). Could Dawn Summers make three? We will see if her pilot, “Mercy,” ever sees a television screen. The show will center around three nurses who share a friendship. Trachtenberg will play a new and inexperienced model. Hopefully no whining!