Traitor Legions w/ special rules for them
ex: Iron Warriors get bunkers or extra tanks, Night Lords give a -1 Ld to enemy troops in CC, Word Bearers get discount Daemons, Alpha Legion has infiltrate, World Eaters have an ability similar to the Maelstrom of Gore apoc formation, Death Guard cause Nurgle's rot, Thousand Sons have a mega-spell or something, Emperor's Children get combat drugs.
obviously costing more points, but would cause some cool themed armies
(yes i do realize some of these are overpowered, but it would be all equaled if my perfect world)