<samp>, <var>, <kbd> are definitely weird ones. Maybe <kbd> would be more useful with some better default styling. I think <output> has some accessibility wins but I've never seen someone use it. <dfn> is a good pick, <abbr> is also up there with questionable semantic tags. <del> and <ins> are basically tailor-made for rendering diffs. <ruby> is really useful for Japanese, doing that styling manually would be a huge pain, but niche for sure. I'm happily surprised <small> survived. I still miss <center>.
I tried the exact same three! Eventually gave up on that path, thinking "I could swear HTML had some way to have a glossary...", and of course seeing it does after revealing the answers
