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Lean burn is not coming back. The only way to pass NOx emissions is to rely on the cat to remove most of it, and cats can't do that above stoichiometric without a dedicated NOx treatment device. Direct injection can allow high AFR because it concentrates the fuel in a smaller area, but that fuel will burn hot and produce NOx.
Cam phasing is alright, but isn't SUPER effective with the small duration cams you usually find on cars. With a longer duration cam, in retarded position you can get extremely late intake valve closures which really allows efficient operation. My guess is that the internal combustion engine's final evolution will be to direct injected, phased cams + variable lift as standard, perhaps a novel multipoint ignition like the Mazda experimental laser plugs, or little to no improvements to the engine and just relying on hybridization to improve the performance.
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