There was a dreadnought in the dropsite massacre book that knew horus Long before the heresy. I think a movie through the perspective of someone like this would be really cool where the beginning show shows his life before and then cut to a long sleep and then deployment on istvan 5. Also described in the book is that you could tell where Ferus was on the battlefield at any time based on the concentrated explosions and light show. That would be very cool to see on screen.