In "Of Mice and Men," what dream did Slim have?
By 'dream' do you mean ambition? Unlike George and Lennie, Slim doesn't feel the need for any kind of partnership or long-term friendship - he is a true loner. Slim is not a 'fleshed out' round character; he is rather a flat character, even a stereotype of the lone male drifter seeking labour out West during the Depression years.
Quiet, disarming, naturally authoritative without turning to force, Slim is the opposite of fist-swinging Curley. He has a position of leadership on the farm and has won the respect of all the workers; he has earned his rightful place. Slim feels at home right where he is; he is biding his time in a limbo land of nonexpectation without even the thought that anything better could come along.
Soft-speaking Slim is held is suspension, so to speak, in a stagnating economy whose inertia affords little hope for anyone's dreams to actually come true. Unlike George and Lenny, he is a realist. His devise would be "bloom where you are planted" and make the most of the situation at hand.
There isn't an absolute answer to this question, but there is an implied answer. Throughout the novel, Slim is placed upon a pedestal and regarded differently than all of the other characters. Everyone else, including the boss, are common, realistic characters with flaws. Not Slim. He is "Prince of the Ranch", able to hit a fly with his whip. Because of his skill as skinner, the other characters look to him as the most intelligent of the bunch, and Slim falls into the role of leader whether he wants it or not. Because he has this distinction, Slim does not have any close friends- he ls larger than life and unattainable. Everyone in the novel is isolated in some manner, and Slim is as well. It hints at a possible relationship with George, but due to the conclusion, readers know this can never happen. So, while it is easy to interpret the dream of every other character, one can only infer that Slim may be lonely and isolated as well, and wish for companionship just as the others do.