Thousand Sons, for as i have said before, if you have the Space Wolves what better way of showing old enemies and bitter hatred and the divide between those that fell to chaos and those who remain loyal to the emperor.
In my opinion the playable warbands should be: Black Legion, Death Guard, Emperor's Children, World Eaters and Thousand Sons. The first four were hearth of treachery. Each one, except Black Legion, has it's own chaos god patron. And Black Legion is the biggest chaos undivided warband. Anyway, I voted for sorcerous Thousand Sons because I hate psykers so they'll suit as my enemies well.
The major problem with this is the fact that GW doesn't own the license for Malice, rather Richard Ford does, whom did some freelance writing back when they didn't have control of everything, which would open GW up to lawsuit. That's why no more Malal lore has been developed and has since then, been retconned
