I had an idea on the train the other day concerning Marks of Chaos in a Warriors army. Right now, I don't think they are doing a good job: Tzeentch's +1 ward save is a little overpowered right now, poor Nurgle's ability is far too highly situational, and Slaanesh is as crappy as always. Granted, it makes me happy as a Chaos player to have a 3+ ward save on my general, but I don't think a Tzeentch character should be a "tank".
I don't have point approximations, but I have some idea of what I think Marks should do. First, I think that Marks should be different on characters versus units. As champions of their dark powers, I would assume that their powers would reward them better than the average Warrior or Marauder. So, here are my ideas:
Mark of Tzeentch:
Unit of Tzeentch: Unit counts as a wizard for the purposes of channeling power die, and they re-roll failed to hit rolls in the first round of combat (Tzeentch's foresight)
Hero/Lord of Tzeench: All the benefits above, and +1 to all casting rolls (also maybe re-roll in every round of combat, but maybe not)
Mark of Nurgle
Unit of Nurgle: 5+ Regeneration, cause fear (I debated between 6+ and 5+, and figured that because flaming attacks are not too uncommon, 5+ was okay, but I suppose a 6+ regeneration would mimic the current Mark of Tzeentch pretty well)
Character of Nurgle: As above, +1 toughness
Mark of Khorne
Unit of Khorne: Unit counts as a level 2 wizard for the purposes of channeling dispel dice and for dispelling, Frenzy
Character of Khorne: as above, Devastating Charge and possibly Magic Resistance 1
Mark of Slaanesh
Unit of Slaanesh: Stubborn (possibly always strikes first, but that seems a little OP)
Character of Slaanesh: as above, and +1 Initiative and +1 Attack for every unsaved wound suffered.
Now, I know that all of these are significantly more powerful than they are at present, but if rumors are true that we may lose the ability to mark normal Warriors and can only mark Chosen, I think these marks go a lot further in terms of making marked units really feel different. I also think Marks should be a per-model cost, (but as is I totally love my 40-man Khornate Marauder horde) and I think that these marks should cost significantly more than Marks do at present. I'm not sure what Chaos Undivided should do.