Here’s my solution. So in the 1850s, there was a smell movement, mostly promoted by future Ag Secretary and Arbor Day Founder J. Sterling Morton that wanted Nebraska south of the Platte River to join Kansas. If this were to happen, I’m guessing Nebraska would be the state stretching from the Platte to the latitude the Wyoming Montana border is at, while Dakota would be the state north of that line.
As a result, Kansas would gain mostly agricultural land, with the largest town being Lancaster ( otl Lincoln) which is a town of about 20,000 and has a small state college but that’s it.
Meanwhile, Nebraska, now including half of South Dakota, had both agrarian and mineral wealth, and Omaha becomes a much bigger transportation help. You also see the Capitol moved closer to the western part of the territory. My money would be on Yankton as it was the Dakota territorial Capitol. Thus it’s a bit larger in otl with about 50,000 people. It also serves as a rail terminus for points west like the Black Hills (now mostly in Nebraska) and points west. You’d still need city leaders in Omaha to do better but maybe with a more diverse economy, they’d do okay.
As for Dakota. It’s mostly just otl North Dakota with towns like Aberdeen and Watertown added. It’d still have oil but that’s it.