The 255 horsepower 3.5L V6 in the all-wheel drive IS 300/RC 300 would seem the obvious engine to power a NX 300, but by the numbers, it only represents a 20 hp boost over the NX 200t.
Adding to the mystery, Lexus has re-registered the GS 300 nameplate in the USA. It’s possible the GS could also be getting the detuned 3.5L V6, but adding another engine this far into the lifecycle would be a curious move and it seems an unlikely powertrain for the next-generation model.
(Do these trademarks signal the development of a new engine altogether? It would make sense, as an updated NX and a next-generation GS could conceivably be introduced in the same year.)