I feel Space Marines should have fallen under the faction "The Imperium of Mankind" and CSM under "Chaos Undivided". This route would leave room for future expansions and regular additions to each faction. ORKs have a ton of classes that can be added and Eldar are very diverse. Heroes and Elites such as Inquisitor Lords, IG Ogryn, Certain Mechanicus units, Kommisars for the Imperium. For Chaos Undivided Daemons/Daemonkin, Renegade Ogryn, Dark Mechanicus, Khorne Berserkers, Plague Marines, Noise Marines, Mutants, Renegade Elites. That's a lot more leeway and room to work in new classes. As far as IG and Renegade IG being playable foot soldiers(non elites) the only viable way to implement them would be player controlled AI squads. Pets per say. They would be extremely squishy compared to the existing factions and would have to make up for that by an IG player controlling a 5 to 10 man squad of NPCs.