Given the option to take multiple detachments in 7th edition (including multiple Combined Arms Detachments), I think one particularly strong option for Chaos Space Marines is as follows:
-Take two CADs, each with 2x Warp Smiths as a HQs
-In each CAD, take 3 Maulerfiends (for a total of 6 Maulerfiends)
-Use Cultist mobs for your required Troops slots and to get/hold Objectives (attach a Warp Smith to each Cultist mob to make it Fearless and give some very solid melee punch, with his 6 x S5 AP2 attacks on the charge)
-Take a few Heldrakes for anti-flyer support (i.e. Vector Strikes) and their amazing S6 AP3 Torrent shooting.
-Add in fortifications (i..e. say two Vengeance Weapon Batteries, for a total of four Battle Cannons) and a Chaos Biker Squad for snapping up hard-to-reach objectives and you should be set.
Overall, this is kind of a Min-Max type list, but it kind of plays on the same principles that made Tyranid “Monster Mash” lists so fearsome back in the day (i.e. 4th edition)…. use cheap, throw-away troops with so many fast moving, deadly close combat “monsters” (in this case, actually walkers) that your opponent can’t deal with all the threats and gets overwhelmed. The Heldrake air support really adds to the whole synergy of this force, as does the Vengeance Weapon Batteries with their 4 x S8 AP3 Ordnance pie plates a turn (very solid against Hordes, MEQs, and even medium-light mechnized lists). Finally, the fast moving Chaos Bikers are relatively affordable and can turbo-boost around all game as your Objective Claimers and QRF (Quick Reaction Force) if any of the Maulerfiends need a hand.
While some armies, like Dark Eldar Lance-heavy forces, can give this battle-group trouble, against most All-Comers lists I think this should do quite well and provide Chaos players with an alternate way of playing that is more melee-centric that there usual options.