I know that you have said you don't want chaos legion crossover, and I agree with it in some cases where a class doesn't fit a legion, but some classes could be used for more than one. Here are some ideas:

-death guard heavy

-night lords sniper

-world eaters bulwark

Also, give every class a blank, customisable chaos armour if they want a legion not yet introduced, like thousand sons or emperors children.