The person that Negan picked up in his car is probably the girl that found out Dwight was on the Hilltop's side. She has been missing since that episode where she escaped the setup ambush by the hilltop. Another possibility is the person picked up by Negan is Gregory, but what would be the point in that? His character has no purpose anymore.
I can't believe it is looking like Negan will still be the bad guy going into next season. They only have two episodes left this season to settle things.
I guess Simon will be the big story for the remainder of this season and possibly Dwight since Negan will learn that he has been working for the other side.