Going back and looking on it, I believe it acctually forshadows a fourth movie. The door being open. Everyone always thought it was to signify her being safe. What if it was actually to leave the door open for another film?
Now that S4 was released and S5 and S6 are in the works as a new trilogy, the open door basically means "This isn't over, Ghostface is still out there, Roman wasn't the true mastermind, someone else is far greater than Roman is still out there, the haunting of Ghostface is NOT over and the past is not at rest.
It's easy to say this now that there is a fourth film, but at the time I think it was meant to symbolize that Sidney no longer had to live her life in fear.