Along with terminators getting to have Marks of Chaos I think that Dreadnaughts should get them too.  I mean every cult would seem to have its own special Dreads that hold the decaying personage of a cult marine.  So here's my ideas.  What do you think?  How about the points cost?

Dreadnaught Marks of Chaos:

Mark of Khorne: 20 points - Khorne Bezerkers Champions that are not killed in battle but have been critically wounded while bezerk are sometimes entombed in a special Khornate Dreadnaught.

Effect: Khorne Dreads have +1 strength and if they have a ccw it is treated as Axe of Khorne (i.e., on to hit roll of 6, get an extra attack). However, an entombed Khorne marine is bezerk and therefore must re-roll any result of 2-5 on the Dread frenzy table with the second roll being the final result.

Mark of Slaanesh:   30 points - Slaaneshi Marines that have been overcome by the pleasures of Slaanesh to the point where they cannot ably take the battlefield can only continue to be used by placing them into Slaaneshi Dreadnaughts.

Effect: Slaanesh Dreads have +1 Attack (however, an opponent should scowl at a Slaanesh Dread not converted with an additional limb (tail, arm, etc.)).  Additionally Slaanesh Dreads re-roll any result of 6 on the Frenzy table with the second roll being the final result.

Mark of Nurgle: 25 points - Plague Marines that get too close to a Nurgle greater demon are often victims themselves of the vile viruses that surround these creatures.  Such Plague Marines can often only be saved by being wired into a Nurgle Dreadnaught.

Effect: Nurgle Dreads ignore glancing or penetrating hits on a roll of 5+ due to the puss of Nurgle that covers their mechanical body (same scowl should be given if green/purple/brownish goo is not covering a Nurgle Dread).

Mark of Tzeentch: 40 - On a rare occasion, a large number of Tzeentch sorcerers are able to come together and entrap the power of the warp in a Dreadnaught.  The basics of the spell are the same used that originally entrapped all of the Tzeentch Marines however it is a much lesser spell and more often than not kills a majority of the casters.

Effect: Tzeentch Dreads re-roll a Frenzy roll of 1 with the second roll being the final result.  However if the result is a 6 the Tzeentch Dread makes a subsequent roll and on roll of 4+ fires twice at the closest enemy model (i.e., does not shoot at friendly models).  Once per battle a Tzeentch Dread can make a psychic test on a Leadership of 10.  If the test is passed, then the Tzeentch Dread can draw upon the warp to unleash Flames of Tzeentch (48", S9, AP2, Heavy 1).  If the test is failed the Tzeentch Dread cannot fire and may not attempt the test again during that game.  The Flames of Tzeentch may not  be fired twice if a 6 is rolled on Frenzy.