That seems a little too far-fetched. Will it be exactly like the book? Obviously not. But it's extremely probable that it'll follow the same general lines, because it's effectively an end to Paul's arc - it would basically end with him going into the desert, blind. Anything else would be way too complex for a single film and leave way too many strings untied. The director already said he's making 3 films at most, and you'd also struggle to keep all the actors available for longer. This isn't the MCU where you can make your entire career about a franchise. These people got shit to do.
There's a little room to expand for sure given that Messiah is fairly compact, but this is still one single movie - not a whole TV season. And they couldn't even get the first book into a single film. They have plenty to fill their runtime without going super crazy and trying to rewrite Messiah or do Children, too.