Are you still doing the "dead fandoms" thing? Because I've got two: the Foundation trilogy (in 1966 it beat out the Lord of the Rings for the "best series of all time" Hugo; a few years ago io9 did a SF series bracket and it lost to BSG in the first round), and Max Headroom.
What I think happened with the Foundation series is that, like Heinlein, who for several generations was THE science fiction writer, it suffered because there stopped being one single way to become a scifi fan, one single path to becoming one.
What I mean is, if you were a scifi fan in 1970, for instance, the way you became a scifi fan was pretty linear: you usually got in through paperbacks that were omnipresent at the drugstore, you started reading the magazines too, which is how the average guy would find scifi for the first time, and realized there was a community around all this weird stuff, you got a fanzine or two or went to the WorldCons or some of the other conventions, and before you knew it, you were telling private jokes about corflu, Ghu, and the Feast of Bosco (all pre-internet fandom memes) alongside Sam Moskowitz, David Kyle, Uncle Forry and the rest of your fellow Slans.
If there’s one path to being a fan, then there’s one core canon of key texts, things “everybody has to read.” That’s what having a unified fan culture does, it says “you have to read this.” Put this at the core of your identity. Just like groups into certain genres of music identify the key albums and songs to their identity that everyone in the group has to know. And that’s why we had Foundation and Heinlein at the center of scifi fandom for decades.
But…what happens when there are multiple routes to becoming a scifi fan? The internet was at the center of this fragmentation. There are people who got into scifi because they loved Valdemar and the Mists of Avalon, and they found other people through the internet newsgroups who did too, and who never even heard of Dune or Stranger in a Strange Land or any of the things fans were “supposed to have read.”
In short, this was how Heinlein went from THE scifi writer and Foundation went from THE scifi series to being just another writer and just another book on a shelf of scifi books.
There are some elements of Foundation too, that make it less compelling to the eyes of modern readers as tastes change. For example, Foundation is a part of a scifi subgenre that has fallen into disfavor: the “God’s Eye View” scifi novel that goes over history in a huge sweep, which often covers thousands of years and entire civilizations in a blink of an eye. The reason why this ambitious subgenre of scifi fell out of favor is obvious: scifi fandom expects characterization to be a part of a scifi story, and the “God’s Eye View of History” scifi story is light on characterization and even characters.
The first and most influential “God’s Eye View” novel is Last and First Men by Olaf Stapledon, which covers all 10 million years of human history. There are other books that tell stories by hitting the fast forward button on history, nearly all of them inspired by Stapledon, who wanted to find the wider themes of human history and by how we’d transform as we enter the cosmos, possibly becoming something entirely unfamiliar or inhuman. Arthur C. Clarke in particular owes a debt to Stapledon.
Stapledon, actually, is a much better candidate for a Dead Fandom than Foundation, since Stapledon is for the most part in total oblivion despite being one of the central figures of early scifi fandom. Like John Carter of Mars, “Last and First Men”are like the grandfathers of science fiction, inspiring the people who inspire everyone else.