Support for ReplayGain (in the web UI)
ReplayGain would greatly help in normalising the audio levels when playing songs from a radio. Quite often a loud track will transition to a track with relatively low audio levels, and vice versa.
Some Subsonic players have support for ReplayGain, by downloading a file completely and then extracting the required meta data. This becomes a problem however in the web UI, as it only has the audio stream and the meta data provided by the API. Adding ReplayGain to the API per track, similarly to things like licensing information or track title could solve the problem I think.
This could also help when transcoding into formats that don't have official support for ReplayGain.