I'd wanna see Horus Rising as a movie with parts from the next book. The ending would be Horus awakening from his coma with an evil glint in his eye...And then you'd have to wait for the next movie.
But out of that, the scene where they're fighting the Interex and Horus asks the Big E why he made him Warmaster and says its too much, that's the scene I want to see the most. The sadness of it would be a perfect note to end a movie on after the uplifting (in my mind) tone the movie would have in the beginning and middle.
Or else Leman Russ tearing Magnus apart and breaking his back on his knee.
Edit: @Pulse; that'd be a great start to the movie, there would be an endless horde of barbaric, massive, terrifying orcs and then the marines would fall upon them and destroy them. It would introduce the audience to space marines and the Emperor if they didnt already know them and it would be a great point to start the Heresy Movie as Horus becomes Warmaster.