Yes, I've read that, and appreciate the desire to maintain a user flow vision. That being said, I don't think adding watch folder support will necessarily create more "tedious workflows".

I also don't necessarily want to save every document I download from online to Zotero, but I don't know that at the time. For instance, I often download a pdf, read or skim it (on my preferred PDF reader, which is also optimized for my purposes, and which can't be easily replaced by the built in Zotero one), and decide I'm not going to use it. I have no need to add that article to Zotero. In fact, if it had already been added, it would just take up space, add clutter to my collections, and get in the way of eventually making an overall works cited for my project.

On the other hand, once I do read it and decide I'll be citing it, then I want to add it to Zotero, and now that I've read it, I know precisely which sub-collection to put it into and which tags to add.

Frankly, I think this is the main reason why the Save to Zotero feature isn't always ideal and why a quicker way to add from a local drive would be more useful. If it were available, I'd use an automator script (I'm using a mac) connected to a shortcut that would automatically port the file into that watched folder and so have it automatically added to Zotero. Bonus point for a workflow in which I download ten pdfs in the course of research, skim them, and decide to keep a handful – select those, hit the shortcut, and they're all in Zotero with minimal effort and a more streamlined workflow.

The open files via Zotero sounds promising, though I'm not sure exactly what it would look like.

We use a watch folder with a different reference manager tool so that multiple lab members can drop PDFs they have obtained into the watch folder, and these are transferred to a central folder and renamed. This makes a central repository of papers that can be accessed by people who are not using a reference manager