It was the opener to a trilogy from the get go.

The "original" Dune was a duology, before it was decided to make Dune Messiah a book in itself and not last part of Dune.

Herbert much later decided to return to the Dune world with God-Emperor, which was a stand-alone in his mind (it became a bridge book onkly after the fact).

He decided later to tell the story of the result to Leto's plans, and that he did conceive of as a trilogy from the start, with the two first volumes published and the third one (which remains open ended in the Dune style, but less so than previous ones) outlined.

If you don't have the patience to suffer through the awfully written two-volumes Dune 7 by KJA-BH to dig up in the dross the concepts and ideas Herbert intended to explore through the ending (they are quite apparent still, especially after having read the other books so recently, though they aren't developped with anything near the intelligence of Frank), just ask and I'll tell you about what I remember. You read so fast that I almost recommend you pick the books, though. I found the ideas interesting enough, even if as novels this is real garbage.

Whatever you do, don't read the prequels, and definitely don't read them first. KJA-BH liberally stole from the outline of Dune 7 to "set things up", and setting things up in their mind was spoiling everything by extending every good idea into elaborate and boring storylines set thousands of years before., as they believed Frank's outline for Dune 7 was "too complex" and required much backstory (which was plainly wrong: Frank would have managed to include everyhting and make everything clear within a 300 or 400 pages book, but he just didn't spoon feed his readers as if they were dumb the way KJA does) What they achieved is that every idea in Dune 7 feels quite stale and predictable once you get there (and I don't mean it spoil the plot, I mean the way they bring them up through the prequels in a very b;ah and pedestrian way sort of ruin the ideas themselves...)