Call it random speculation while bored at work - but could Magnus the Red (the primarch of the 1000 sons chapter) have been an attempt by the Emperor to create a primarch spliced with the Navigator gene? I could see the advantages to that experiment....a primarch that could see the warp and use it to guide his actions, but remain uncorrupted by what he saw. Or at least better protected anyways. If Magnus hadn't been lost to the Emperor I wonder how it might have otherwise turned out....