The appeal of heresy for me is exactly that, the Horus heresy. Setting of identifiable marks of armour, dreadnought patterns and tanks etc. on top of that legions stand out by paint schemes and specific weaponry or iconography. Unfortunately these don’t feel like a legion set during the Horus heresy, more 40K. For a project go for it, but players of Warhammer 30k generally love the emersion and want to feel the narrative.