Just to note, on going Renegade/Heretic. It'd be very fluffy if your chapter had internal strife once the player makes that choice. Whatever the reasons, when it comes to leaving the Imperium, battle brothers within the Chapter are rarely in lock step about their opinions of it. It becomes a real test of faith and loyalty; whom do you truly serve? Your chapter and your chapter master, or the Imperium and the God Emperor?
While this is a great idea, at the same time I feel this should also come about when you are able to split off smaller chapters from your own into new foundings.
That way you can include renegades splitting from your chapter, loyalty divided grumblings and those leaving as a result, chapter chaos (not of the God's variety), and of course creating new successor chapters.
With modifications based on what sort of chapter you are (Dark angels still being interconnected, Black Templar just creating new crusades etc.)