A Jadeveon Clowney / Javon Kearse type athlete to play alongside AD. Otherwise, teams will continue to triple team Donald with no consequence. They also need a linebacker who is a pass rush threat.
The Rams must present the threat of deploying at least three devastating, athletic pass rushers at a minimum. These players must be talented enough to be on the field at all times.
The threat this year of Donald, Fowler, and Matthews was good, but I think a major weakness of the Rams' defense was that the best athletes weren't always on the field on early downs.