To chat a bit about my own home state, as Wisconsinites have an intensely strong sense of regional identity, Milwaukee missed several opportunities to become a greater metropolis than it eventually did.
The first is the easiest, and is a fate that could have been achieved by several other Midwestern cities, such as St. Louis or Cleveland. Milwaukee could have potentially ended up as the rail hub of the Midwest, and therefore the rail depot of America. Much like Chicago its potential for commerce and industry is also boosted by its position on the western shores of lake Michigan. If this was switched, expect the fate of the two cities to be more or less reversed.
Going on to a later date, with Chicago’s regional primacy well established, Milwaukee still stood out as the German American Athens. This granted the city a great amount of prestige as far as Germanic immigration was concerned. However WWI and the immigration restrictions following kind of ended that era of the city’s history. If WWI gets butter flied away, is shorter, or involves a neutral America; Milwaukee will be one of the destinations of choice for German and Austrian immigrants. This could very easily mean another one or two hundred thousand people by 50’s
To go even later, Milwaukee’s borders got more or less frozen during the 50’s. Milwaukee made a ploy to gain sole access to the water rights for the land and suburbs to be, surrounding the city. When the initial move failed, it went to the state legislature, where it was defeated by a single vote, from a Milwaukee legislator who happened to be a rival of the mayor. These moves were unpopular, and it helped galvanize the suburbs against former expansion. Without it, well greater Milwaukee land could have easily become a city of a million
Finally one could remove the biggest instigator of white flight, was a singular race riot in the 60’s. While the trend was probably inevitable, it was a straw the broke the figurative camel’s back. It was the big trigger to a surge of white flight which ultimately led to the urban sprawl of Waukesha. Without a riot, the decline certainly would have occurred, but will be smaller. It would also remove a lot of the racial bad blood in the city, and its environs.