FabFilter User Forum
Pro-L3 idea: Spectral limiting and priority EQ section
One idea for potential FabFilter Pro-L3 is spectral limiting, which might use the same FFT approach as FabFilter Pro-Q4's spectral dynamics but with psychoacoustic model (derived from lossy codecs like MP3 and AAC) to shape the gain reduction per frequency (e.g. masked parts received less gain reduction or something else)
Another idea for Pro-L3 is the priority EQ section, which could behave superficially similar to Pro-C3's sidechain EQ section, which depending whether spectral limiting is enabled or disabled, either changes how the spectral limiting process prioritizes certain frequencies (boosting these will limit these frequencies more aggressively and cutting these will "protect" certain frequencies at the cost of more reduction on other frequencies) or in case of spectral limiting being disabled, the distribution of faster transient and slower release stage activities per-frequency (boosting these frequencies will make the slower stage more active on these frequencies, or cutting these will make the faster stage takes over more on some frequencies)
BTW, the Pro-L3's spectral limiting idea is not complete without "Spectral Density" settings from Pro-Q4's spectral dynamics that adjust number of scalefactor bands used for spectral limiting, as well as gain reduction spectrum display on the priority EQ section of Pro-L3 that is absent in sidechain EQ section of Pro-C3 (even on "TTM" style that is actually multiband)
Alternatively, the priority EQ could work similarly to Waves L3-16 multimaximizer plugin when spectral limiting is enabled (to make it more consistent with priority EQ on broadband limiting mode/spectral limiting disabled); boosting a frequency will "protect" certain frequencies at the cost of slightly more pumping and cutting a frequency will allow certain frequencies to be attenuated more with less pumping, especially when psychoacoustic model is either more sophisticated than or differs from ones used by lossy codecs (e.g. MP3, AAC, Ogg Vorbis and Opus) right?
On that note, I've somewhat implemented the idea of priority EQ (for FabFilter Pro-L 3) on my own multistage limiter effect (as a sidechain prefiltering for the first stage, which uses K-weighting though priority EQ in Pro-L3 could be anything):
codepen.io/TF3RDL/pen/emBzNpm
BTW for FabFilter, feel free to omit the spectral limiting in favor of something else (e.g. more intelligent channel linking using an iterative numerical solver algorithm similar to DMG Limitless that works out gain reduction across multiple discrete channels on both stereo and especially surround formats) if you don't need it