I think they’d do a bit of a “catch-up” segment at the beginning, which ends with Jessica doing the ritual to become the Reverend Mother (and accidentally fucking up her unborn daughter in the process).
They’ll then time-skip, start off with an introduction to Feyd-Rautha after maybe a scene of Rabban fucking it up on Arrakis, and go from there.
Well, Villeneuve has said, if both movies do well, he’d also like to make Herbert’s second book, Dune Messiah. That book, just like Dune Part 1, is very much about the journey of Paul Atreides.
You could probably consolidate Dune Messiah and Children of Dune into a single long movie. The SyFy miniseries did that, and it makes sense – it’s basically one long story of Paul rejecting the choice to follow the Golden Path, while his son picks it up and runs with it.