Hordes of Chaos
Back during the early days of Warhammer Fantasy, all three types of Chaos servant - Vikings in cool armor, anthropomorphic animals with swords and axes, and creations of the Chaos Gods - were all part of a single army called the Hordes of Chaos. The army played like how the current Warriors of Chaos army did, relying on powerful, expensive close range units with VERY strong army leaders. In order of the power and cost, from lowest to highest, the ranking went; Beastmen, Warriors, and Daemons. But as time went on, Gaming Workshops saw that more armies, provided they were selling, meant more money since they could make more models and rulebooks, so they split the beasts off into an ineffectually army that accomplishes nothing (in fluff and on the table top!). Seeing that complete lack of success, they did it again by spinning off daemons into a separate army, and reason decided to change the name "Hordes of Chaos" to "Warriors of Chaos" since the daemons were removed, though the warriors of chaos were pretty much the focus of the book in the 6th edition even with the presence of the daemons.
So we have three Chaos armies, and none of them get anything done because not even the Chaos Gods have the power to violate the status quo. PROGRESS!
But Games Workshop may finally be realising that the current split is awful because, apart from Warriors, none of them are really strong enough to carry themselves as independent armies. So they created rules for Legions of Chaos as part of The End Times: Glottkin splats.
The Hordes of Chaos approach was surprisingly simple, even elegant. Certain kinds of units were designated as inherently Special or Rare units - generally the various gribbly monsters - but the rest of the units were divided into three racial categories; Mortal, Beast or Daemon. When you selected your general, units of the same racial category were Core choices, whilst those of different racial categories counted as Special choices. Daemon Princes could take either of two cheap upgrades to be able to take Mortals or Beasts as Core units, reflecting their former origins amongst the living.