work for multi-source was done in experiment/multi-source but has been scrapped as the changes are far too complex for me to properly review, and there are UX issues that come with mixed queue (no play queue sync, no playlist saving)
however, as described in the PR, I think it would be much simpler and easier to implement an account/server switcher. This is what most other clients that I've used do... this would not support mixed queues (songs from different sources in the same queue) however the entire change would be much less complex. the behavior would probably be simpler to the user aswell
this would still likely require a database migration and structural changes but it shouldn't be as painful to implement
i don't think i'll implement this unless asked, because i personally have no need for this feature myself. but if you would to see this added, let me know by reacting/replying here!