Well, the last books of the original series of Dune take place over thousands of years when Leto II becomes the worm via symbiosis with the sandtrout. There is so much that happens in these millenia, sometimes told in flashback through the everlasting Duncan Idaho, or through the narrative of the religious texts.
The wars and the scheming and the plans and the plots... I mean just the sisterhood's rise and attempts to undermine leto could be its own series. Heck there are more subplots in Dune than fables in the Silmarillion. In fact, I think that the flaw with the Silmarillion is that everything becomes so much of the same after a while that I stopped caring about all of the one-off stories: it got repetitive and, frankly, boring.
Dune did suffer a Jon Galt moment with all of the exposition in God Emperor of Dune, but there was a lot of storytelling in those meetings between the Duncans and Leto.
