The center fuel tank on the XLR's caused a delay in design. Not regular NEO's, but the XLR's.

Yes, the 787 was designed to replace the 767. But even the 787-10 really isn't big enough to carry what the 777-300's can do. It is close to a replacement for the 777-200's as currently configured. But also doesn't have the seating capacity of the A's. So close to a doable replacement, but not quite. If Boeing were to increase the weights on the 787-10, then maybe for the 200's. But then you are squeezing more people into a smaller box.