I remember seeing on the surf.to/orders website that UPS was seen as a potential 744F customer, so I'd expect them to go for that.
However, I agree that the 777F is inevitable. It's much the same size as the MD11, which was highly regarded as a freighter (well, it had to be good at something) and now that this is out of the way, the 777 is a natural successor. Given the room for growth in the freight market, particularly in Asia, it's only a matter of time. I'd expect SIA, Malaysia, JAL, Korean Air, Asiana and United to be among the lead customers. Maybe if KLM went for the 777, Martinair might follow and replace its MD11s with 777s as well; it also operates M1Fs. (I'm just watching a Martinair MD11 video at the moment, so that comes to mind).
Hopefully, for many current MD11 operators, the list of interested parties won't include FedEx; I'm sure most current MD11 operators - SR, KL, AZ, are hoping FedEx will find a home for their aircraft!