Clean-sheet engines might imply a few things... Higher cruise speed and different integration concepts (above wing, integrated into tail, etc) are just the start. It'll add risk and development time but if they really want to push the product out ahead of the tech curve they're gonna need to pull out all the stops.
A mini-787 with squashed dual-aisle cabin and engines above the wing or one integrated into the tail for wake energy transfer, or whatever, might do it but would still be an evolution of what we've got already. All of the NASA-sponsored concepts under investigation (like BWB and distributed propulsion) are too far out in the future I think.