This is a very good idea. Slashdot hit on something very good with their scoring and moderation system, and then the lessons were forgotten in the systems that came after Slashdot.
There are so many small tweaks like this tagging idea that would improve things, that I think the way to do it is a frontend “plugin” system that can accommodate them without it being a big fandango of tweaking the core to include fifty of these little nubbins all over the codebase.
Nothing that’s ordained from on high will ever be completely perfect. If you can have a little text box on your custom frontend, where you can tweak your own UI features, and then talk with the instance admin about importing one of those little tweak plugins for the whole site, or for your own community only, that sounds like a huge step forward to me. If you’ve ever added little tweaks to Wordpress, with the custom CSS boxes or by adding a few lines to functions.php, I envision this being similar.
This is all much more ambitious than I was thinking of when I asked this original question, but it would be a lot of fun to work on. If the result was it going back up into Lemmy, and each instance having its own collection of tweaks and a thriving community of people working on them without needing to disrupt the pace of development on the core, that would be a pretty excellent outcome.