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IMHO the next steps should be (if still subsonic):
1. Extensive use of additive manufacturing with hexagonal sandwich honeycomb structures (metal)
2. Fly-By-Light (optical fibers) instead of Fly-By-Wire, Power-over-Fiber, higher redundacy
3. Ultimate range capability (ie. At least 10,820 NM = 1/2 of the Earth's circumference) to be able to land everywhere in the world from any point
4. Mach 0.99 cruise speed with supercritical airfoil
5. Large ailerons (50% of the wingspan), stabilators, all-moving tail, boundary-layer control, aerogel for thermal and acoustic isolation,
6. Cabin that is pressurized at sea-level (14.7 psi) cabin altitude at all altitudes.
7. Large windows, parquet floor, memory-foam leather seats (32" wide x 43" pitch), LED lighting
What else?