Hi,
Thanks for the details, it helps a lot!
Do you mean displaying terms from different taxonomies (like categories, tags, product categories) mixed within a single drop-down?
I’m afraid that is not possible to do – even by changing the templates or by making a hook to force displaying terms from different taxonomies, as the filtering on the server side would still not work, as the taxonomy would be incorrect for the terms.
I usually recommend a custom code where I can, but in this case this is not possible without completely changing the core plugin code.