Do you think that to become a CSM you will have to start out as the lowest ranking SM and through some sort of option in a main quest/side quest you are able to switch to Chaos? I think that would be awesome! You would be able to see your armor change/degrade by the presence of the warp and it would allow friends who are on opposite factions to be able to play together in a way that isnt killing each other! What do you guys think?

I've thought about this too, and agree that it'd be great for numerous reasons.

For one, it just makes sense from a lore perspective. Obviously it wouldn't be so cut and dry as choosing your side if this were to happen in the actual universe, but from a game standpoint it would be more of a representation of corruption rather than a realistic depiction of how it works.

Secondly, and more important (in my opinion), is what it would do for game balance. Space Marines on both sides are no doubt going to be the most popular classes/races/specializations when the game launches, but the non-Chaos version is likely going to outnumber the Chaos version by a substantial amount. What the developers could do is nudge people toward the lesser populated version.

Most games with good/evil or light/dark choices tend to make the evil choice equal to the good choice, resulting in almost everyone being good. With W40k, however, Space Marines are evil too. It wouldn't be too much of a stretch to make Chaos equal or even more attractive as non-Chaos. As long as it doesn't negatively impact a given player's desires (which is something to think on), the faction balance would greatly benefit from such a system.

I also think it would be interesting for this situation to happen multiple times the higher level you get and the deeper you get into the game. It would define HOW your character fell from grace and joined Chaos and would also describe what weakness caused them to do it. Like a certain mission is based upon loyalty, and you have the choice to break loyalty. Or maybe there is one based on greed and your character leaves because of some sort of dark promise. Not all missions will be so cut and dry though, it may be you make the decision to join chaos because of something that your character believes is wrong.I think that every new opportunity that your character is presented w ith to switch sides will define your characters bio even more!

While this would be cool. It would be simpler to use the methods described in the Ultramarines Omnibus and standard recruiting procedures. The whole turned traitor thing is kinda not realistic because individuals who go rogue usually don't end up in a CSM warband. Warbands have their own recruiting techniques and some are even "Vat" grown.. or grown inside grotesquelly fat human females who serve as berthing vats. I really don't want to see a gross berthing chamber like that in the game... But I think it would be easier if they just introduced you as a CSM neophyte who jus got out of beat down slavery school.

The other method is like you're discussing... spend the first quarter of the game as a loyal servant of the Emperor.. then turn traitor... Just that not many warbands just take in rogues... they'd sooner kill them or take em as slaves. There'd have to be a good plot for this.

Heres an answer:

So maybe you could start out faithful to the emperor and after you have chosen to side with chaos, your gene-seed could be taken and implanted into your new Chaos Space Marine! I know that it would be taken by force, but it could be just another way for you to side over with Chaos. Either way, I just hope that Vigil has enough forethought for that type of gameplay change, while at the same time making it balanced, so that people might actually WANT to play CSM!

Using the gene-seed of a SM to breed new ones doesn't pass on his consciousness. The devs would have to make stuff up for that to work. Going way out of their way just to explain why you can be a CSM is not proper game development.

It was just an example, a wrong one as you have pointed out , but it IS needed of Vigil to make it appeal to other people because every normal person who comes into 40k ONLY knows about Speesh Mareens.