This sounds like a school assignment for you!
In order to approach this task, I recommend you take two characters as they are and give them one more trait or dimension. For example, if George was just a little craftier and willing to put up with Lennie for longer, he might have grabbed his things, met up with Lennie, and the two of them would have run to the nearest town to grab a train to somewhere else. If, for example, Candy had rolled up a little more of a stake, maybe the three of them could have been out of town by the day that the whole group went to town during chapter 4 leaving Lennie and Crooks back at the ranch. It only takes altering one or two roles just a little bit to imagine that things could have been entirely different.