Prominent actors/actresses who have never been in a sequel?

Oldman was also in Hannibal, though his character doesn’t appear in Silence of the Lambs or Red Dragon.

No, she’s been in a series of TV movies, Prime Suspect

Sorry, he’s was in an Animal House sequel.

http://www.imdb.com/character/ch0010133/

Is this what you’re calling the sequel to Animal House?
Where Are They Now?: A Delta Alumni Update
A mockumentary that was a bonus feature on one of the DVD releases?

Yes, why not? It may not be a movie with a theatrical release, but it’s still a work of fiction in which he acted a part. And the part he acted was a reprise of the character he played in the film. Why wouldn’t this count as a sequel?

Brad Dourif? Yes, he was in RotK, but not in the theatrical release.

A mockumentary not even released on its own, but as a bonus on a DVD doesn’t come close to a “sequel” in my book, or in the eyes of most people, I’ll wager.

He appeared in both the rebooted Halloween and its sequel as well as all five Chucky movies (voice mostly, but he was on-screen for 1 and 4).

As you wish. I’m not going to argue.