FD1 - Two characters in FD5 are riding the Flight #180 plane. They're supposed to be dead. That means two people were in the same chain with Alex and Clear that we don't know about. This is unlikely but if the writers wanted to bring back Alex he could have faked his death somehow. It'd be less cheap then him getting killed between two movies.
The third dude might also be alive, somehow. He was alive in the original filmed ending where Clear gave birth and broke the chain.
FD2 - Kimberly and Cop dude are still alive.
We've learned how all the FD2 characters were supposed to die, but the FD1 characters survival changed the timeline. This could be true for all FD Deaths.
FD3 - Wendy and the other two rollercoaster survivers and a few people on that train are possibly still alive.
Kimberley was originally written to ALSO be on this train so it could be the end of FD3 could be another vision.
FD4 - There are possibly more survivors in the big stadium. Hopefully they were more bearable.
Everyone in the Mall. There's a magazine printout with background on some other characters but I really don't care to rewatch that movie.
FD5 - People on the Bridge. The FD5 cast could have made other people fall and hit the water early / later then they were supposed to. Because of the death design there are people who were severly crippled from the water hit but couldn't die until their turn in the second runthrough.
There might be some I missed, these just came to mind