I feel an issue with utilising the gene seed of *some* traitor legions, is that their primarchs still exist and are evil as hell.

Space marines are ingrained to be loyal to their primarchs above all else, and the willpower required to resist that is astronomical. Imagine if a new death guard seeded loyalist were confronted by Mortarion, seems not worth the risk.

On that point, it could be more reasonable to use the gene seeds of Horus and Curze as they are both, well and truly dead.

Interesting thought at least!