We know the last scene will involve Ghost!Jamie looking at Claire in the window, in Inverness, before she travels through the stones. DG said she's had the last scene written for 15 years.
My theory is that Jamie predeceases Claire, but his spirit stays with her for the rest of her life. His spirit isn't bound by time, so he roams throughout time, and watches himself with her in different moments of their life. It also means he watches Bree grow up in the 20th century. If this is how it ends, Ron and Sam had to be told because they'd need to shoot additional angles of Sam watching scenes that are being filmed now.
In the last scene, where he's looking at Claire's window, when Frank speaks to him, he turns around because Ghost!Claire shows up and calls to him and they disappear together into the mist. The End.