Add a "--bitrate-per-channel" or a "--quality" argument
As of now Opus doesn't have a setting equivalent to Vorbis, Nero AAC, or QAAC's VBR quality levels. This makes it difficult to batch encode multiple files with multiple channel configurations. With a quality level you can pick one setting and the encoder will adjust the bitrate according to the channel layout: for example, QAAC @ q127 will target 320kbps for 2.0 channel and 768kbps for 5.1. This is a very handy feature.
If a --quality feature, may I suggest a naming scheme that has some relation to the bitrates output at 2.0? I mean, I'd have no idea what Vorbis' q=5.0 translates to without looking at a chart. QAAC, OTOH, seems to somewhat correspond to bitrate-per-channel in its q45-q91 range. This is pretty useful. I know to pick q91 if I want ~180k quality without needing to run off to a manual.