Next generation is indeed a vague term, but I would use HEVC as a good example of a next generation codec. For another codec to be next gen, it should have the

Next generation is indeed a vague term, but I would use HEVC as a good example of a next generation codec. For another codec to be next gen, it should have the same or better compression than HEVC: about half as many bits to encode the same video than H.264 required.