Except for bigger, better more efficient engines : Although Raptors are a great achievement, their design if used alone is not made for bigger loads. SpaceX is

Except for bigger, better more efficient engines : Although Raptors are a great achievement, their design if used alone is not made for bigger loads. SpaceX is stacking them now for larger ships. Hopefully the whole system will prove reliable on higher flights, and a different design won't be needed.