773, 77W, 77L, 77-8, 77-9 then throw 78-8/9 in
I guess by the time a 787 could fly in EK colours, there will be no more terrible 773's hovering around. Same goes for the mighty 77L's.
Most probably a fleet with 788/789 plus 778/779 will be in force. That's basically 2 types with each 2 different lengths. No problem at all for MFF, just a matter of certification, meaning money. But this would be put off so fast with the efficiency.
MFF for 350 and 380 might be achievable, but i would fancy very expensive. Let's not forget that between a 2-holer and a 4-holer there are extensive operational and performance differences.