I've only read up to Children, myself. I didn't find Messiah to be all that different from the first one, though; it almost feels like a "10 years later" epilogue to Dune to me, so I do get Denis kind of envisioning these two books as one complete trilogy.
Children, on the other hand, feels like a full on sequel with a tone/style all its own. And from what I understand of the next three, those do as well. If Children and beyond ever get adapted, I don't think Denis would really be the right fit for them (nor do I think he has any interest beyond Messiah).