A monologue from Curley's wife's perspective might be enlightening. She might comment upon "all these men" and upon her perspective on the opportunity-limiting Depression. She might analyze and comment upon both George and Lennie from an objective, exterior perspective. Analysis of her character might produce a very interesting monologue.
In Steinbeck's novella, there are many a monlogue. Take, for instance, George's recitation of how he and Lennie have each other in Chapter One. After having looked at his and Curley's wife monologue about how she could have been an actress, you may want to compose one by Crooks, employing some of the ideas he expresses in his conversation with Lennie about aloneness and not having someone against whom he can "measure" himself.
It would be interesting and challenging to use Curley's wife as the source for your monologue. She is one of the most lonely and misunderstood characters in the book. Her talks with Lennie would be a good source.
I would like to see more about Lennie's character. Lennie's conversation is very limited (given George is constantly telling him to keep his mouth shut). I am sure that much is going on in Lennie's mind, readers are simply kept from his thoughts--based upon his limited mental capacity and George's constant warnings.
To me, George is the most interesting character in the book. He is almost continually placed in a compromised position. Most of his choices carry considerable moral weight.
He chooses to help Lennie escape justice for what he did in Weed. He lies to make sure that he and Lennie can get a job. He warns Lennie to stay away from Curley's wife and Curley, and later pushes Lennie to be violent with Curley. Finally, he is forced to a very difficult decision when Lennie kills Curley's wife...