Wouldn't make sense unless for chaos Even then a daemon prince would aggro grey knights and grey knights are too OP for the game
Either Angron will fuck you with his chain axes because he's mad all the time, Fulgim will torture and kill you for pleasure and Mortarion will turn you into a plague ridden pile of infected meat because it's inevitable. Yeah fun times.
The only 100% for sure dead primarchs are Sanguinis Horus Ferrus Manus Then we have Guiliman in stasis Then there's a chance that Alpharius is dead, but after Guiliman killed him in his vanity he burned the body and now nobody can confirm whether or not it was actually a primarch. Otherwise they can return. But not in this game. Please not in this game. Please.
I think the issue is more that no one knows if it was Alpharius or Omegon that was killed, since the Primach's were twins and that fact was known only to them and possibly the Emperor.
Or if it was some random alpha legionare. Either way it can't be confirmed because guiliman was vain :l