I don't think anyone will ever truly replace OU. I'm one of the few Husker fans that do not like Iowa as Nebraska's main rival. It is not a game I look forward to, and they are not a program I care about at all.
When NU first joined the B1G, I was really hoping that Penn St. would be the Black Friday game. No one cares about Nebraska-Iowa the day after Thanksgiving. It is not a nationally relevant game and does nothing for Nebraska's brand. However, it definitely hurts Nebraska's brand every time they lose that game. Which they have regularly over the past several years. And it greatly annoys me.
I am really hoping with the new additions to the conference, that Nebraska can get something going with either UCLA or Washington, since NU has a long history playing both schools.
But Oklahoma will always remain the top rival.