Prefer providers and manual selection
I'd like to be able to actively prefer one provider over another, as some tend to provide consistently better subs than others. Also a list of available subtitles per file and a manual forced-download would be really awesome though I don't know whether that's possible as a Plex addon right now.
I'm afraid all of your feature requests are not possible at this moment. To give more preference to a provider is something that cannot be done in Subliminal by only using their API. This would require a feature in Subliminal to provide an order of preference which is included when comparing the best subtitle. It's best to file a feature requests there, once it is in, I can include it in the addon.
A list of available subtitles is possible, but this means that we download all subtitles from all providers (now we only download the best match). But there is no way of giving feedback to the user what the best match is (in a Plex client). And we will use more bandwidth (downloads) on all sites, instead of just querying them.
What do you mean by a forced download exactly? When the Plex agent runs (on a metadata refresh) the contents is downloaded at that moment. That content is then available to the user in a Plex Client. I think you mean by having a selection of available subtitles and downloading one of those? Unfortunately this is not the way the plex metadata agents work right now.