Before you start shrieking hatred at me, please read my reasoning for what I'm doing below. I'm having my 6 players' characters very slowly mutate into becoming primarch-like beings.Your head's already exploded, hasn't it?
Here's why I want to do this, based on the fluff of the 40k universe. The emperor was created when all of the shamans on Earth decided to unify their souls into one being. I've read five of the Horus Heresy books, including the one where Horus looks into the past where the primarchs were created. The book indicates that not only is the emperor's soul special, but the primarchs' souls are made of some non-standard stuff too. If I'm reading it right, the book implies that the Emperor actually captured some of the raw power of the warp and used it to supercharge his 20 sons (21 including the twins). Otherwise, the primarchs simply would have been normal men. Now, if the souls of the primarchs are similar to the emperor's own, then why couldn't they reincarnate if they could escape the clutches of the ruinous powers?
The idea is that my group's characters are descendants of the primarchs. Why not? The emperor had kids too. Because of the resonance their souls generate with the primarchs' own, they can stand out in the warp as a beacon. On a fateful journey through the warp, the geller fields of the MIsericord failed and the primarchs' souls darted to attach themselves to the player characters' souls.
Before you jump to any conclusions, no, the characters do not start out special, nor does the soul fusion provoke any amazing transformations. Nor are the characters' personalities subsumed by the primarchs'. I'm planning to have the characters very gradually mutate throughout the sessions so that they only approach primarch-likeness at rank 16, with 45,000 experience points under their belts.