In an upcoming campaign, I'd like to try something new, and I'd appreciate some thoughts or ideas on how it could work.

In short, the player characters are being heavily scrutinized by the four Chaos Gods: every action, decision and bad thing that happens to them has a chance to impress them and raise a tracker of sorts.

This is obviously inspired by a new tracker found in Omens of War...

Ideally, the tracker would be composed of four tracks (one for each god). Any time something happens that would make the relevant god happy, its tracker would go up.

Also, keeping in mind that the gods are rivals, there may be chances that when one achieves a certain popularity, the rival tracker may start to decrease.

The problem I'm facing is: what actions would be appropriate? Some, like Khorne, are relatively simple, while others, like Nurgle or Tzeentch may require a bit more thought. I figure that a tracker may go up when a PC is very successful in certain tasks, roleplays in a certain way or suffers from certain debilitations.

For one thing, I'd imagine that a tracker could go up whenever a PC succeeds really, really well in a particular kind of Skill or Action that is in line with the relevant Chaos God's ideology. Conversely, the tracker may also go up when the player generates Chaos Stars in those same kinds of skill checks.

The other issue is: what happens when the tracker gets to a certain level? I'd like to find a way that the Chaos God begins to whisper promises of power and temptations. Perhaps offering an extra expertise die once in a while? Maybe once the tracker gets to a certain point, the hero must make a Willpower test to resist the lure?

What do you think? Again, my goal is not to PUNISH the players, but to give them an interesting internal conflict to deal with.

Sounds like you're looking to start a lot of bookkeeping

For one, I'd go with something along the lines of a stance-bar

Khorne <--x--> Slaanesh

Nurgle <--x--> Tzeentch

Overal Corruption (either by corruption tokens, or as a progress-bar)

Promises/Gifts/... are "achieved" regardless of what god you align yourself with, they're all detrimental to society.

As for what they are: Generally, I make them skills/action cards that are RAW unavailable to the character, but highly useful.
For Tzeentch, give them the ability to use a specific spellcard, but only once every session, use the Hedgemagic rules for that card.
For Khorne, no-brainer, go for endurance and or damage. Drawback: discipline checks to avoid stress, temporary insanities.
For Slaanesh, pain/pleasure: give them options to willingly suffer pain/wounds/...access to drugs/potions/... to get a bonus. Only have them make increasingly difficult discipline checks to avoid becoming addicted. When addicted, slap em with penalties UNLESS they do the nasty. If they nastify themselves twice, give them the bonus as before. This can be used on social as well as combat abilities btw. Set up their downward spiral bit by bit :)For Nurgle, ability to infect people - with a great big smile!, but also an unrelenting vigor, high agility, great willpower, downside: the people around them suffer the consequences through illness and infections. And that does include partymembers...

As for what actions would influence this: I would say if you keep with the 2 antagonistic pairs, it makes your life a little simpler. Plus, it gives the player 1 tracker that's fairly obvious (Khorne/Slaanesh), but one where you as the GM have a lot of leeway: Tzeentch is unfathomable after all! Keep a player guessing and they'll post questions/comments about it on their facebook for days after the session.

I'd simply let the first random mutation belonging to a chaos god determine it. But you don't have to worship a chaos god to be a mutant. You could be a devout sigmarite and still mutate and keep your faith while the world shuns you.

Chaos worship is a deliberate act... mutating is not.

While specific action cards, characteristics and what not can certainly work I would probably start with the bigger picture.

What are the general ideas that each chaos god personifies?

Khorne = slaughter, murder and violence for no other reason than violence itself

versus

Slaanesh = sensations and drive for perfection

Nurgle = stagnation and perseverance

versus

Tzeentch = knowledge and change

Regarding Black Crusade FFG wrote this on the four gods:

Though these charges are undeniably true, they are only facets of a much more nuanced picture. Khorne is the God of bloody slaughter, but he is also the god of martial pride and honour, setting oneself against the most dangerous foes and winning against the odds. A devotee of Khorne is as likely to be an honourable champion in combat as a blood-crazed slaughterer.

Slaanesh is the God of hedonism and excess. But this is true in all things, not just carnal pleasures. Those who desire to indulge in the finest culinary delights, the most beautiful artworks, even the most sensual clothing, could all be amongst Slaanesh’s disciples. Just as importantly, Slaanesh is also the god of perfection. The singer striving for the most beautiful song or the warrior who seeks the perfect fighting techniques, both could be devotees of Slaanesh.

Nurgle is the god of death and decay, to be certain, but he is also the god of rebirth. After all, decay is simply one part of the cycle of life, without which no new life could grow. In the same way, Nurgle is also the god of perseverance and survival. While those who wish to spread decay and corruption are certainly amongst his followers, there also those who wish to endure, to become tough enough to handle the threats of an uncaring universe.

In many ways, Tzeentch is both the best and least understood of the Dark Gods. Almost everyone knows he is the God of Fate, plots, and schemes, as well as the god that exemplifies the ever-changing nature of the Warp. However, Tzeentch does not plot towards some end (at least none that can be comprehended); he schemes simply to scheme. He is constantly building, even as his devices unravel under their own complexity. At the same time, he is the god of knowledge and comprehension, and his devotees may be those who seek an understanding of an enigmatic universe.

http://www.fantasyflightgames.com/edge_news.asp?eidn=2028

I do not necessarily agree 100 % but I think it is good enough as a starting point.

Since the Reckless podcast 9 im tinkering with a idea of build a "X" with the corruption tokens. The middle is the general corruption of the character, being in each "side" of the token connecting another corruption token for a dark god, building a big "X" track.

Interesting idea. Might be relevant for which type of mutation to "grant."

jh