I definitely agree that Nebraska might feel "their brand" has not grown or benefited from the Big 10 move, but I don't believe for one second that it means they want to return to the Big 12.
Realignment which was not done as well as it could have, has tarnished the Big 10 a bit. Nebraska was a good FB addition. However watering down everyone's home schedules with Rutgers, Purdue, Indiana, and Illinois the last few years has hurt the league. Iowa had a dog of a home slate last year, and Nebraska's is nearly as bad. Pitt was pretty much Iowa's marquee home game last year. Last time I checked, Pitt is not in the Big 10. Oregon played that role for Nebraska this year. On the flip side Iowa gets Wisconsin, Michigan, and NU this year so it evens out, but Iowa will probably have a horrendous home slate again next year.
I do think NU fans would not admit it, but deep down they probably miss the villains in the Big 12. The evil Texas empire, the historic rivalry with OU, the pain in the ass Bill Snyder led Cats, those wacky Tech offenses. Things were chippy between the NU and other Big 12 fanbases. Who besides Iowa does the average Nebraska fan REALLY want to put a whoopin' on?? Maybe Wisconsin? They probably don't give a rats pajamas about NW, Purdue, Illinois, and Indiana this year.