First order of business has to be someone who keeps us up. That doesn't exclude someone who is progressive, but that is secondary for now.
That means someone with a bit of experience of this situation, and success in dealing with it.
Generally, in these situations, you need someone with the opposite personality of the outgoing manager. The biggest thing that struck me about Hammell, at least in his interviews, was how intense and serious he was. That's not a bad thing to be for part of the time, but at some point you need to be able to impart some fun and enjoyment in the players, even when they are underperforming.
So whoever comes in, as much as anything else, needs to bring in a fresh approach and mentality.