In the 300-350 seat segment Boeing promotes the 787-10 and the 777-8.
The 777-8 will probably have excellent payload-range performance, able to fly over 9000NM. At a costs, it will probably be 25t-30t heavier (OEW) than the same sized 350-1000. The 787-10 seems best suitable for flights up to 5000NM.
Biggest reason for this is the size of the 787 wing. Optimized for the 787-8, enhanced for the 787-9, it leads to high wingloads and fuel capacity limitations on the 787-10.
In my opinion, it is therefore likely that, in time, Boeing well develop a new bigger wing for the 787, enabling bigger improved engines too.
Kaktus Digital - Boeing 787-11X
Two versions, a 787-10 sized and an -11x stretch variant would offer better direct competition to the A350XWB, which took over many 777-200ER/300ER operators, like BA, LH, AF, UA, AA, DL, CX, SQ and JAL.
Kaktusdigital made some great artist impressions.
