I don't have access to any privileged information, here. My best bet is that DARPA is lighting a fire under JPL's ass to explore variable-cycle engine technology: https://en.wikipedia.org/wiki/Variable_cycle_engine

The US has tested ramjets before, but mostly in the context of missiles and boost-glide vehicles. The "holy grail" of efficient propulsion would be to taxi/accelerate with a turbofan engine, bypass into a ramjet for Mach 3-to-6ish, and then transition to a liquid-fuel rocket motor for the last Mach 6-10 climb. An engine like that could escape the Earth's atmosphere without sacrificing efficiency at lower altitudes. I realize that "space fighter engine" is also a longshot, but China has ramped up investment in variable-cycle technology quite a bit in the past 2 years. Simply solving the turbofan-to-ramjet transition would be a huge boon for high and fast flight.

Issacman's quotes in that article suggest, to me, that he wants to reboot the SR-72 program (or something similar) with cleansheet propulsion as a feature. The SR-71 could be the engine testbed, and a future airframe could be designed around the requirements of the engine. Just my $0.02 though.