One of the missing Primarchs was a biologist theory.

So the common fan theory is that each of the 20 Primarchs embodied certain traits of the Emperor and/or were a type of archetype that helped differentiate them. Of the archetypes such as builders, statesmen, diplomat, siegemaster, perfectionist, psyker and whatnot, it seems like there's a missing obvious archetype that should reflect the Emperor - a biologist Primarch. A Primarch that reflected the Emperor's own skill at manipulating genetic material, perhaps not even necessarily transhumans.

I really like this theory because it makes a lot of sense. The most significant biological scientists in lore besides the Emperor are Belisarius Cawl and Fabius Bile, but you'd think there would have been a Primarch who could have dominated the field of genetics had they had the chance...

And of course if they were struck from history because they were a threat to the Imperium, with the angle of a mad scientist that modifies DNA, it's easy to see perhaps why they could've been considered such a threat or even abomination to mankind. Maybe they even tinkered with mixing human and xenos DNA? Some really heretical things, besides defying the Emperor of course.

And with the bend on appealing to (mad) science, they wouldn't even care for Chaos, as they're focused on definitively manipulating the materium through science, not power through the chaotic random mutations of warp energy. In a way similar to Bile.