In no particular order, some things I have been used to and found useful:
Add History inside the Find field (F3) inside the Viewer. This is quite useful when searching for few terms across files. Also it is good reminder when coming back to this later.
When Ctrl + F (search text in file) DO can automatically drop the string inside the Viewer Find's field. This way, you don't need to copy / paste, or worse type/mystype that string when you want to find it inside one of the files.
Navigate Back & Forth when the current file list is collection (?) i.e. Find, Filter or something. I tried the "Show / (empty) / FILE / AUTOFILELIST/ LISTSIBLINGS", command but neither combo works. You have to select the files then show the Viewer.
Strictly Read-Only mode, like without the caret navigation. If you need to select something, you can just use the mouse. The Up / Down arrows can be used to scroll across the page instead.
For the standalone viewer, the next/prev list is automatically generated for image files when you double-click one.
We assume people don't want to use our standalone viewer for things like text files and documents (unless they ask specifically via the Show command with the files selected) since there are usually better programs to open documents in. (Not talking about the preview pane, where you don't want another window to open at all.)
Hmm.... I use the standalone Viewer for quick glance and navigation at whatever I can throw at it, mostly text documents, but also and images.
Biggest benefit it is, it is fast and consistent, there are much more fancy text editors, but they are usually heavy and tailored just for texts. Same logic we can apply for any other type of documents - use a dedicated tool, however this means you need to fire up something more complex and switch interfaces. That's not enabling a "flow of work".
If there was an generic and lighting fast standalone previewer I would use it instead as replacement. So far, I don't think I have found something. File editors usually have ones (like DO) which cover this requirement.
@ryobg have you tried QuickLook - it's designed to replicate Quick Look on the Mac and allows you to preview the selected file by tapping the space bar
If they implement Microsoft's Preview Handler API (the same one File Explorer and Outlook use for viewers) then it should work in the Opus viewer pane. Otherwise, probably not.