Have you considered just grabbing a Primaris or a primaris character and chaosifying them?
The blood angels lieutenant could look very cool as an emperor's child. He's got the pose, sword, and even hair for it. You'd have to do something about the blood angels icon on his chest but keeping some of their old imperial stuff and defacing it can add a lot of character to chaos models. Perhaps a big crack/scratch through the blood drop to make it unrecognizable as a
BA specific chestplate and rather just an old imperial chestplate. Then the sword arm shoulder pad is a separate part and totally interchangeable, as is the backpack and head, so all of that can be simply replaced with chaos parts. The gun arm could be replaced with any other marine arm (though it might need to be primaris/new chaos sized). Perhaps that's where a Death Guard tentacle arm and a generic chaos shoulder pad can come in?