I would like to see a feature to normalize the audio of a whole track/multiple clips or at least when exporting the project as video file. My clips have different volumes. Adapting them manually is too much effort. From what I've read, ffmpeg provides audio normalization features.
Such effect already exists, you'll find it under Add Effect -> Audio Correction -> Normalize in kdenlive.
Audio mixing is part of the editing work. There are several normalization filters (also based on normalized loudness criterion)