"What's the odds on Hagels replacement being a democrat?"
Anonymous sources and John McCain. On FOX News.
If you want informed speculation, you'll have to take it off the spin cycle.
http://www.npr.org/2014/11/24/366379250/hagels-successor-may-face-a-tough-nomination-process
".....Well, first and foremost is Michele Flournoy. She served in the first three years of the Obama administration as the Pentagon's undersecretary of defense for policy and she currently runs a defense policy think tank here in Washington that she co-founded. She's publicly been supportive of the administration's policies in Afghanistan and fighting the Islamic State, which could be problematic for Republicans.
Another contender is Ashton Carter, who left the Pentagon as deputy defense secretary shortly after Hagel arrived. Whoever is nominated, Hagel plans to stay on until that person is confirmed....."
Think it is a given it will be. Someone with little to no real experience.