Likely candidates right now are British Airways, Lufthansa, and perhaps Japanese carriers.
But Boeing hasn't got a final configuration for it yet. They recently decided to make the passenger and freighter the same length due to better than expected aerodynamic performance in wind tunnel tests. Boeing had been shooting for 8,000+ nm range for the passenger version, but wind tunnel test showed it would really have an 8,300 nm range. Several airliners told Boeing they'd rather have less range and more seats with lower CASM than more range. They also said having the same fuselage length would increase the residual value of the planes when they are later converted to freighters.
Emirates, however, is upset that Boeing won't build the shorter longer range version that was initially offered. Supposedly Boeing is considering ways to decrease enough weight on the 747-8 by substituting redesigned composite parts for existing metal parts for to give back the 8,300 nm range with the longer stretch. If Boeing can pull that off, it will drive more nails in the A380's coffin. The 8,300 nm mile range would be very useful for Emirates to fly to the west coast of the US. The decreased empty weight of the longer stretch 747-8I would drive down CASM even more for airlines that don't need extreme range while increasing the performance of the 747-8F.
There was news that there might be a few orders for the 747-8I maybe from Lufthansa at the end of December.