I'll start by saying three things:
1: I don't have a single clue of how one would go about getting a new model into the game.
2: I don't know if making mods online compatible will ever be possible.
3: tl;dr: I don't know anything about Space Marine modding.
With that out of the way, ever since i saw that there were no horned Chaos Helmets for Space Marine, and that the Chaos aesthetic in the game is so weird i wanted to make my own, whether anything new can be put in game or not is beyond me, but considering we have Santos and Copernicus both working on stuff i assume it will be possible eventually, either way i'm still working on DoW II stuff, which is something you'd know already if you read the last few posts on my DoW II thread, so worst case scenario nothing i make for Space Marine gets into Space Marine and instead ends up in DoW II even though it'd be ridiculously high poly in comparison(not that i haven't done that in the past already).
So onto business, here's:
Absolutely nothing, this is going to end as a horned Chaos Marine helmet but right now it's just a space marine helmet(from the game) cleaned up a bit to remove as many triangles as possible. Just wanted to point out that i'm at least going to try and do something for Space Marine.
Cross fingers, toes and other bodily appendages that you can cross, not that we want to know what else you can cross.