I think the 2 missing Legions are supposed to be from 2 Primarchs who were never found. Like Cerrakoth said GW specifically left them out for people to have fun with on their own. In Dawn of War the Blood Ravens make the statement of "for the Forgotten Primarch!" or something along those lines. What I would like is permission to use the creature design rules to make a Primarch for use with a Space Marine army. 
