I thought that it would be a really cool idea, and something that would maybe spice up the repetitive gameplay by having a mechanic that would allow someone to invade random pve matches as one of the enemies whether it be tyranid or chaos. I believe that this would help with the monotony and help spice up the game for everyone. You could make it a setting in the operation selection to have invasions enabled, and if you are an invader let the person pick which enemy to be.