It will be 777X or A380
No. In the quite near future the 380 will be phased out. The race will be between the 350 and the 777X.
AB has to give its 12 wheeler -1000 wings and prove it can haul as much as predicted. Up to 350 pax the 350 seems very good, a threat to the 77W.
That's the main reason Boeing is "mulling" a stretch of the T7: It wants to occupy the 400+ region where the fuel guzzlers were roaming. If it gets dominance there, the family concept can sell the shorter T7 versions vs. the 350.