If Mytrle hadn't died in The Great Gatsby, then Tom wouldn't have had the chance to convince Wilson that Gatsby had anything to do with Myrtle's death (when he claims Gatsby is the driver of the yellow car that struck her down).
Wilson would lose the plot and become a murder after despairing about Myrtle's death, and Gatsby would also have no reason to fear that Daisy might take the blame, which would have altered how he proceeded.
In short, it would be a completely different story as Myrtle's death is a significant plot point.
Have a look at this clip to get some ideas about The Great Gatsby if all the deleted scenes from the movie, and how an alternate ending could have been pieced together from clues in the novel and deleted scens from the movie.
A must watch for anyone interested in Gatsby!
