Reeling from the loss of their eldest son to the war, a family welcomes the company of a mysterious soldier claiming to fulfill a promise to a fallen comrade.

Who is the main character? The father? The mother? Or the returned soldier?

Since this logline is a little vague in parts, some specifics might differentiate this from The Guest. Or there's room to change parts that give the whole a fresh feel, like a female protag, another kind of family, etc.

This sounds very similar to the plot of The Guest.

I would recommend having one protagonist whose perspective we see this story from. Give us the logline from this character's perspective.?

Inciting incident - what is the incident that throws the protagonist's life upside down? I'm assuming that, since it's a thriller, something dramatic happens that kick starts the real story.

Goal - when we have a protagonist, give them a clear visual goal that is their answer to the question posed by the inciting incident.?