As a person who grew up in Missouri, I have gotten to know cities around the Midwest pretty well. I went to school in Springfield MO. It's a city of about 160k that has had modest growth but unlikely to hit 200k anytime soon, especially with the rapid growth in the suburbs to the south. That said, Springfield has become quite the regional center. I would love to see it expand to 250-300k. I would prefer they do more in the central business district.
Columbia Missouri finally surpassed 100k in the latest census. They have alot going on there, but it's mainly a college town with the University of Missouri being the main employer. One thing I could never figure out about that place, is the fact that it's only about 10 miles from the Missouri River. Why on earth did it not become a river city is beyond me. However, Jefferson City, a city of 41k, is right on the river and it's about 25 miles south of Columbia.
Other cities I would like to see become major centers, or maybe larger than just regional centers would be, St. Joseph MO., The Quad Cities, Sioux Falls, SD, Topeka KS, Cedar Rapids IA, Boise ID, and Billings MT.
Speaking of St. Joseph, it's been said that early on, if St. Joe had gotten the railroad over Kansas City, that it might have been the regional center instead of Kansas City.