Based on discussion in other threads (particularly the one about the nature of the Amulet), here are some style guidelines and elements of the universe that should stay consistent across chapters.

1. Remember to capitalize 'Alicorn Amulet' when you are referring specifically to it.

2. In mindscapes/mental projections, the Amulet should usually appear in disguise as a pre-existing pony. This can be a living pony like Celestia or Twilight or whomever, but can also be historical characters like Starswirl. After all, if the Amulet is 'just' an amulet, then it wouldn't have any pony-sona or anything.
2b. This doesn't preclude the Amulet itself from showing up in visions alongside its disguises. Of course, this Amulet is still just an object, and can't talk or take action.
2c. Just because the Amulet appears in disguise, doesn't mean its disguise is perfect or without tells/flaws.
2d. Of course, if some other form is going to make manipulating the pony easier, then that is certainly feasible.

3. Try to make your chapter title a single word that references what the pony is being tempted by. (ex; I'm going to name my Luna chapter "Redemption" and my Cadance chapter "Family".)

4. The Amulet can lie, but it needs to remain consistent during each chapter. It is ultimately manipulative and self-serving, but it can take on many roles and tones in service to its goals.
4a. The Amulet still needs to adhere to the rules of reality. But of course, since magic exists, that doesn't limit it as much as one might think.

Feel free to discuss these guidelines, disagree with them, or propose additions and changes. After all, this is a group project!

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The audience never knowing the Amulet's true motives is all well and good, but do we? Should we?

5965582 given the diversity of opinion so far, I haven't seen enough consensus to be sure. But if we can come up with something to make the story more consistent behind the scenes, I'm all for it.

I suppose we could take all the ideas so far and smush them together, since it will be hidden behind a layer of vagary and implication anyway. But some of those ideas contradict each other :P

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I went with "survival" in my chapter, thought I'll need to change it if we aren't allowed to say what it wants.

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On point 2, does it have to take on pony form in the mindscapes? It has complete control of the scenario, I'd think it could be whatever it thought would accomplish its goals. And if it does take on pony form, I'd think it could appear as anypony, even a new one. If it does make a ponysona, that wouldn't have to be consistent across chapters, as it would be making an Avatar based on the temptee's expectations, not based on its own nature.
On point 3: Progress.
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I agree that we should have a more set in stone idea of the goals, so as to not accidentally make it so its goals are contradictory or null over several stories.

Once we know its actual goal, we don't necessarily have to expose what that is in our chapter. Each chapter might present the characters with a different view on the Amulet's motives based on the tactics it used. Unless the Amulet speaks on its own behalf, the true motives and goals may never be known by the readers, it would all just be conjecture.

But an overarching set of goals or directives prevents one chapter from supposing the goal is "Amulet wants all of the cupcakes" versus another chapter that supposes the goal to be "Amulet wants none of the cupcakes."

5965727 Does it have to take on pony form? probably not. Appearing as just an amulet, or a dog, or minotaur, or whatever else is certainly feasible.

It is because it can appear as whatever, that I think making a ponysona is counter-intuitive. After all, there's no guise it could create that would be more trustworthy than a pre-existing pony. Who is Twilight more likely to listen to? Celestia, Starswirl, or a non-descript gray alicorn she's never met?

It also allows more opportunity for drama. A Celestia or Rarity or whomever else that is slightly off is going to capitalize on the Uncanny Valley, in a way that an original incarnation never could. And if this unsettling quality is subtle enough--perhaps a red tint to their eyes or a strange speech pattern--it also gives the audience the satisfaction of noticing it.

5965577 This is going to be obvious, but I can't resist having the Amulet take the form of Twilight while in the company of Moondancer. The reason behind this is not merely manipulative, since after having read Solaris (great book, by the way) I have become very fond of the idea of characters being confronted with manifestations of people they have either lost or can't be truthful with. The Twilight form is uncanny in how slightly flawed it is, but this would be meant more for tragedy than horror. It's gonna be one of the sadder chapters.

Probably going to title the chapter "Love." Can't think of anything else that fits more.

5974192 As I mentioned in my reply to Moosetasm, having the Amulet appear as a non-pony is perfectly feasible. My main issue was mostly with the idea of the Amulet appearing as pony!Amulet or similar.

The idea behind point two is that the Amulet should appear as whatever makes it the easiest to manipulate its target. Most of the time, that is going to be someone that the target knows and trusts. But not always.

5975012 We already discussed the idea of the Amulet appearing as a faerie/sprite and pretending to be on Limestone's side, and I still think that's a reasonable interpretation of its powers. I don't know if you've come up with a different approach in the meantime, though, so there's plenty of other ways it could be handled

5989302 I've been quoting that old writing cliche; "as long as it needs to be." My Cadance chapter is currently ~3,700, and my Trixie epilogue is ~1,300, so that's already a wide variance.

Focus on making your chapter the best it can be. If that means being 4,000 words or 400, don't worry about it too much