My idea is creating countdowns for different orders as a way of measuring how close they are to rise against the Dominion. This way, it's not clear who will be the arch-betrayer and the rebellion could go in many different ways. Even a dodgy order can stay loyal and a "good one" turn traitorous.
In the Horus Heresy, there are a number of checkpoints that make the plot develop. If things go slightly different (for example, the Ultramarines decide to join Horus side), the whole history changes.