^I think the problem with that is that "Part 1" would be little more than an extended prologue that actually has very little to do with anything that happens in parts 2 & 3 and only one character appearing in all 3 parts...after a fashion at least since it's a different Duncan between 1 & 2 and then it's all the Duncans in 3. For one thing the actor playing Duncan would ave to be different in Part 2 as he's a much younger character. Fudging his age would destroy the father/son juxtaposition with Duncan and Teg as they essentially swap roles in Chapterhouse.
What you'd probably have to do is either cram GeOD & Heratics into one film, either with a LotR style extended prologue that touches on Leto, the Scattering and the Bene Gesserit's finally stepping forwards befor getting on with the actual plot or as I suggested before, weaving the essentials from GEoD into both Heretics and chapter house in the form of flashbacks and narrative back story.
I just can't see God Emperor
working as an introduction to a trilogy on it's own. It's a slow burning book about introspection that focuses on the lonely agony and boredom of immortality and prescience. Not the most exciting way to kick of an epic space opera about the rise and fall of empires and the fate of all humankind...