Kiffin is on record saying he wants to still coach. I doubt he retires willingly… However-- he could stay on as a consultant or take a front office role in some capacity, which would still effectively give the reins to Marinelli.
Rod is a great coach and DC. I think he would be my choice to take over and would stand the greatest chance of fixing the D the fastest b/c he knows the personnel on the roster, knows what we need, and is a master motivator and is able to get the most out of his players.
There isn't another guy out there that IMO-- would be a big upgrade over Marinelli if we're staying with the 4-3.