Building depth in the offensive line is the only way to get them to where they need to be. They need to continue to bring in good offensive lineman (scholarship or walk-on) and develop them.
In the glory days of Husker football, you rarely saw and offensive lineman play before he was a junior. That is why they called it the Pipeline, because they kept developing these guys and they were ready to play after about 3 years. This is one reason Scott Frost wants his roster to be at 150+ players.