>>534958980
Open Heretic doesn't make sense to begin with in a game like this for this same reason why you can't play as Xeno Rogue Trade. Or why you couldn't play as Orc in hypothetical Fellowship of the Ring game. Not everything fits because settings have restrictions. Black Crusade exists for a reason.
Heretical path should be far more subtle and a tax you get for your naive or too opportunistic choices which slowly might push you into heresy. Like when you dress as Chaos cultist on Rykard or when you spare some cultists, not nuke Rykard, not properly purge Kiava, let Hann Cassini live with his crew and sometimes when you do needlessly cruel or retarded dogma choices even.
But honestly even Genestealer cultists playthrough would be easier to put into the game. You would just get a kiss and then had to lure your companions for a kiss. And then most of the game could stay mostly this same with some unique options to corrupt others and spread your cult.