In Of Mice and Men, what does the death of Curley's wife foreshadow?

Lennie's final isolation is foreshadowed by Curley's wife's death. The young woman who shares her faded dreams with Lennie and her sense of frustrated solitude dies at the hands of a man who has his own dreams. As Candy realizes right away, that dream dies when Curley's wife dies and, for Lennie, this death will result in being cut off from society for good. 

We can see the idea of Lennie's impending exposure to a world outside of society at the end of the death scene in the barn. After Lennie partially covers Curley's wife with hay, he hears the sound of the horseshoe game and "For the first time Lennie became conscious of the outside." Immediately, he begins to recall the escape plan and tell himself to go hide in the brush, evoking his early notions of running away to live in a cave. Lennie is no longer part of society.

While this ultimate isolation takes the form of actual death for Lennie when he is killed by George, part of the power of this turn of events is in its surprise. So, although Curley's wife's death foreshadows Lennie's fate, it does not explicitly indicate Lennie's death. Rather, the murder foreshadows the death of a dream in a way that lets us see Curley's wife's "disappointment speech" as applying perhaps to the whole cast of characters in the tale. 

When she is killed, we know immediately that Curley's wife's death will trigger a few predictable responses and will shape Lennie's doom. The reader understands that George was aware something like this could happen. That is why the he and Lennie make a contingency plan early in the story, agreeing to meet by the Salinas River if anything should go wrong. 

Curley's animosity toward Lennie now is provided with legitimacy, making Lennie a real target of violence.

Lennie himself realizes that the hope for owning a ranch with George and Candy is now lost. This is what he worries about as he reaches the river and waits for George. 

In the odd conversation that Lennie has with himself in his fright (as he talks to a giant hallucinatory rabbit), Lennie worries that George will be upset with him and beat him. 

The rabbit speaks to Lennie's harsh, socialized awareness of what he has done, warning Lennie of the worst. 

"If you think George gonna let you tend rabbits, you're even crazier'n usual. He ain't. He's gonna beat hell outa you with a stick, that's what he's gonna do."

Lennie replies with self-reassurance, saying that George wouldn't do that because "he ain't never raised his han' to me with a stick. He's nice to me."

In this instance, we have a complex and ironic piece of foreshadowing that suggests George will do what is "nice" or right for Lennie when he arrives. In shooting Lennie, George is arguably proving Lennie right and proving the giant rabbit right as well. 

There is no more safe place for Lennie in society now. And this is effectively what Curley's wife's death means for Lennie.