What you want IINA to do:

Would be great to be able to add ReplayGain support natively to the UI (I know we can set values using MPV advanced settings but it would be nice to be able to set and alter ReplayGain settings in the audio settings).

Why you think this should be added:

It would allow libraries of music which have ReplayGain tags added to music files to be able to be played at the intended gain levels.

Examples of other projects that have something similar:

Foobar2000 Mac has an excellent ReplayGain implementation, with clipping support etc. Note, I'm not suggesting Iina supports creation of ReplayGain tags, only reading and using existing tags.