I don't think ND would land a good replacement in January. I'd generally rather have an interim coach than make a long-term mistake on some dildo like Fitzgerald. But next year's team really could be decent, and that would be wasted with an interim coach.
Dan Mullen is the name that seems to come up most often. I'm not sure about Dan Mullen. He's outpacing Mississippi State's norm, in a great conference, but 2014 is the first and only year with the success we'd want. In his fourth and fifth seasons, he went a combined 15-11.
Still, while he wouldn't be my first choice (Malzahn would be), I'd probably hire Mullen if possible. If not, I think I'd use an interim coach and see what the pool looks like in 2015.