Codex Space Marines =
any recently turned renegades, or perhaps the Astral Claws/Red Corsairs during the Badab War and the early years after they were defeated by the loyalists. Also some interesting options in there for say an Alpha Legion force!
Codex Space Wolves = Khornate themes as mention above. Blood Claws are also an interesting unit to represent berserkers. The lower
WS being attributed to a more bloodcrazed & frenized combat style that involves just blindly trying to rip your opponent limb from limb.
Could also represent any basic chaos marine force by not taking things like thunder wolves or the more 'modern' Imperial tech.
IW's could get away with using things like Razorbacks since they actively seek out and try to repair anything they can salvage and turn on Dorn's lapdogs!
Codex Blood Angels = any Khornate or Slaaneshii force. Blood crazed or 'Fabio in armour' approches work well. A Baal pred w/flamestorm cannon would be a cool 'daemon engine' varient. The nipple armour is perfect for those fopish emp's children and things like
FnP devies w/plasma cannons which can 'count as' blastmasters are rather in character for them.
Codex: Templars = good basis for any fanatical chaos worshipers like the Word Bearers. You could even go so far as to count the Neophytes as summed daemons who fight in amongst the chanting brothers?! Chappies/Dark Apostles feature rather heavily, those
BA's can field more of them somehow.
Codex Grey Knights = any type of radical inquisitor + minions,
LatD type force that mixes human worshipers/soldier with marines, any kind of heavily psychic chaos marine army (like anyone aligned to Tzeentch!), and such.
That's just some ideas of the top of my head.
Really, no one in their right mind would complain if your chaos army uses a different codex because the current one is so awful and bland as fething hell.