Could be interesting.
For the delays, I can’t tell from the documentation if negative delay is allowed. If it is, what about a single slider for “Stereo width” with a tooltip explaining it’s a delay? The idea is that if a user wants 34ms of separation, then the left channel could be -17ms and the right channel could be +17ms. The “Stereo width” slider would push one negative and the other positive at the same rate. The hope is that the “center” would still be synced to the video it came from. Otherwise, a 34ms delay of the right track would put it an entire frame behind the video if it’s 30fps. Since I haven’t tried this against synced video, I’m not sure which would be worse… audio that’s 17ms early, or audio that’s 34ms late.
Or, maybe it’s best to give the user full control with L/R sliders, and a text note in the filter panel that tells them they can do a +/- manual split as an option.
I’m also wondering if it’s worth giving access to the middle_source parameter. A lot of wireless lapel mic receivers and wired mics will send a stereo signal to a 3.5mm camera mic input, even though both channels are the same thing. So a user might get tripped up by “Mono to Stereo” because technically the source is stereo (dual mono). Or sometimes one channel is hot and the other is a -12 dB safety track. If the user needed to choose whether they want to widen the hot or -12 dB track, then providing a Source option could save them some Channel Copy filters.
The other parameters are probably unlikely to be used, and there are existing Shotcut filters to mirror their functionality if absolutely needed. I don’t think there’s enough unique functionality in those extra parameters to warrant a second Advanced filter.