Some "didn't make it in" things that looked promising were the perceptual vector quantization[1], and a butterfly transform that Monty was working on, IIRC as an occasional spectator to the process.
The primary benefit of PVQ is the side-information-free activity masking. That is the sort of thing that cannot be judged via PSNR and requires careful subjective testing with human viewers. Not something you want to be rushing at the last minute. After gauging the rest of AOM's enthusiasm for the work, we decided instead to improve the existing segmentation coding to make it easier for encoders to do visual tuning after standardization. That was a much simpler task with much less risk, and it was adopted relatively easily. I still think it was the right call.
[1] https://datatracker.ietf.org/doc/html/draft-cho-netvc-applyp...