True but this is already available.
"We" would need something newer and better to top some A321neo with today's latest GTF and PIP by a few percent engine wise. Possibly something with huge bypass ratio and diameter? If a new engine can be integrated from scratch more efficencies will be gained. CFRP wings (maybe a high wing layout?) and fuselage and these new engines with some smart cockpit might make it work one day. I'd say before the late 2020s no new engine like the one needed will be available.